Deformation of press-fitted metallic resurfacing cups. Part 1: Experimental simulation

The interference press fit of a metallic one-piece acetabular cup employed for metal-on-metal hip resurfacing procedures was investigated experimentally under laboratory conditions in the present study, in particular regarding the cup deformation. Tests were carried out in cadavers as well as polyurethane foams of various grades with different elastic moduli to represent different cancellous bone qualities. The cadaver test was used to establish the most suitable configuration of the foam model representing realistic support and geometrical conditions at the pelvis. It was found that a spherical cavity, with two identical areas relieved on opposite sides, was capable of creating a two-point pinching action of the ischeal and ilial columns on the cup as the worst-case scenario. Furthermore, the cup deformation produced from such a two-point loading model with a grade 30 foam was similar to that measured from the cadaver test. Therefore, such a protocol was employed in subsequent experimental tests. For a given size of the outside diameter of the cup of 60 mm, the cup deflection was shown to be dependent largely on the cup wall thickness and the diametral interference between cup and prepared cavity at implantation. For a relatively thin cup with a wall thickness between 2.3 mm (equator) and 4 mm (pole) and with a modest nominal diametral interference of 1 mm, which corresponds to an actual interference of approximately 0.5 mm, the maximum diametral cup deflection (at the rim) was around 60 microm, compared with a diametral clearance of 80-120 microm between the femoral head and the acetabular cup, generally required for fluid-film lubrication and tribological performances. Stiffening of the cup, by both thickening and lateralizing by 1 mm, reduced the cup deformation to between 30 and 50 microm with actual diametral interferences between 0.5 and 1 mm.     

Elastohydrodynamic lubrication analysis of metal-on-metal hip prostheses under steady state entraining motion

The elastohydrodynamic lubrication problem of metal-on-metal hip joint replacements was considered in this study. A simple ball-in-socket configuration was used to represent the hip prosthesis. The Reynolds equation in a spherical coordinate was adopted for the fluid-film lubrication analysis, to account for the ball-in-socket geometry. The corresponding elastic deformation was calculated by means of the finite element method in order to consider the complex ball-in-socket geometry as well as the backing materials underneath the acetabular cup. Both the Reynolds and the elasticity equations were solved simultaneously using the Newton-Raphson finite difference method. The general methodology developed was then applied to a recent experimental prototype metal-on-metal hip implant. It was shown that the backing materials underneath the acetabular cup had little influence on the predicted contact pressure and the elastic deformation at the bearing surfaces for this particular example. Both the film thickness and the hydrodynamic pressure distributions were obtained under various loads up to 2500 N. The predicted minimum lubricating film thickness from the present study was compared with a simple estimation using the Hamrock and Dowson formulae based upon an equivalent ball-on-plane model and excellent agreement was found. However, it was pointed out that for some forms of metal-on-metal hip prostheses with a thin acetabular cup, a polyethylene inlay underneath a metallic bearing insert or a taper connection between a bearing insert and a fixation shell, the general methodology developed in the present study should be used and this will be considered in future studies.     

Importance of head diameter, clearance, and cup wall thickness in elastohydrodynamic lubrication analysis of metal-on-metal hip resurfacing prostheses

The main design features of metal-on-metal (MOM) hip resurfacing prostheses in promoting elastohydrodynamic lubrication were investigated in the present study, including the femoral head diameter, the clearance, and the cup wall thickness. Simplified conceptual models were developed, based on equivalent uniform wall thicknesses for both the cup and the head as well as the support materials representing bone and cement, and subsequently used for elastohydrodynamic lubrication analysis. Both typical first- and second-generation MOM hip resurfacing prostheses with different clearances and cup wall thicknesses were considered with a fixed large bearing diameter of 50 mm, as well as a 28 mm diameter MOM total hip replacement bearing for the purpose of comparison. The importance of the head diameter and the clearance in promoting elastohydrodynamic lubrication was confirmed. Furthermore, it was also predicted that a relatively thin acetabular cup in the more recently introduced second-generation MOM hip resurfacing prostheses would be capable of improving elastohydrodynamic lubrication even further.     

Steady-state elastohydrodynamic lubrication analysis of a metal-on-metal hip implant employing a metallic cup with an ultra-high molecular weight polyethylene backing

The elastohydrodynamic lubrication (EHL) analysis was carried out in this study for a 28 mm diameter metal-on-metal hip prosthesis employing a metallic cup with an ultra-high molecular weight polyethylene (UHMWPE) backing under a simple steady state rotation representing the flexion/extension during walking. Both Reynolds and elasticity equations were coupled and solved numerically by the finite difference method. The elastic deformation was determined by means of the fast Fourier transform (FFT) technique using the displacement coefficients obtained from the finite element method. Excellent agreement of the predicted elastic deformation was obtained between the FFT technique and the conventional direct summation method. The number of grid points used in the lubrication analysis was found to be important in predicting accurate film thicknesses, particularly at low viscosities representative of physiological lubricants. The effect of the clearance between the femoral head and the acetabular cup on the predicted lubricant film thickness was shown to be significant, while the effect of load was found to be negligible. Overall, the UHMWPE backing was found not only to reduce the contact pressure as identified in a previous study by the authors (Liu et al., 2003) but also significantly to increase the lubricant film thickness for the 28 mm diameter metal-on-metal hip implant, as compared with a metallic mono-block cup.     

A general axisymmetric contact mechanics model for layered surfaces, with particular reference to artificial hip joint replacements

A general axisymmetric contact mechanics model for layered surfaces is considered in this study, with particular reference to artificial hip joint replacements. The indenting surface, which represents the femoral head, was modelled as an elastic solid with or without coating, while the other contacting surface, which represents the acetabular cup, was modelled as a two-layered solid. It is shown that this model is applicable to current total hip joint prostheses employing ultra-high molecular weight polyethylene (UHMWPE) acetabular cups against metallic, metallic with coating or ceramic femoral heads as well as metal-on-metal combinations. The effect of cement is also investigated for these prostheses using this model. The use of a metallic bearing surface bonded to a UHMWPE substrate for acetabular cups is particularly examined for metal-on-metal hip joint replacements. Both the contact radius and the contact pressure distribution are predicted for examples of these total hip joint replacements, under typical conditions. Application of contact mechanics to the design of artificial hip joint replacements employing various material combinations is discussed.     

Analysis of elastohydrodynamic lubrication in McKee-Farrar metal-on-metal hip joint replacement

An elastohydrodynamic lubrication (EHL) analysis was carried out in this study for a typical McKee-Farrar metal-on-metal hip prosthesis under a simple steady state rotation. The finite element method was used initially to investigate the effect of the cement and bone on the predicted contact pressure distribution between the two articulating surfaces under dry conditions, and subsequently to determine the elastic deformation of both the femoral and the acetabular components required for the lubrication analysis. Both Reynolds equation and the elasticity equation were coupled and solved numerically using the finite difference method. Important features in reducing contact stresses and promoting fluid-film lubrication associated with the McKee-Farrar metal-on-metal hip implant were identified as the large femoral head and the thin acetabular cup. For the typical McKee-Farrar metal-on-metal hip prosthesis considered under typical walking conditions, an increase in the femoral head radius from 14 to 17.4 mm (for a fixed radial clearance of 79 microm) was shown to result in a 25 per cent decrease in the maximum dry contact pressure and a 60 per cent increase in the predicted minimum film thickness. Furthermore, the predicted maximum contact pressure considering both the cement and the bone was found to be decreased by about 80 per cent, while the minimum film thickness was predicted to be increased by 50 per cent. Despite a significant increase in the predicted minimum lubricating film thickness due to the large femoral head and the thin acetabular cup, a mixed lubrication regime was predicted for the McKee-Farrar metal-on-metal hip implant under estimated in vivo steady state walking conditions, depending on the surface roughness of the bearing surfaces. This clearly demonstrated the important influences of the material, design and manufacturing parameters on the tribological performance of these hard-on-hard hip prostheses. Furthermore, in the present contact mechanics analysis, the significant increase in the elasticity due to the relatively thin acetabular cup was not found to cause equatorial contact and gripping of the ball.     

Explicit finite element modelling of the impaction of metal press-fit acetabular components

Metal press-fit cups and shells are widely used in hip resurfacing and total hip replacement procedures. These acetabular components are inserted into a reamed acetabula cavity by either impacting their inner polar surface (shells) or outer rim (cups). Two-dimensional explicit dynamics axisymmetric finite element models were developed to simulate these impaction methods. Greater impact velocities were needed to insert the components when the interference fit was increased; a minimum velocity of 2 m/s was required to fully seat a component with a 2 mm interference between the bone and outer diameter. Changing the component material from cobalt-chromium to titanium alloy resulted in a reduction in the number of impacts on the pole to seat it from 14 to nine. Of greatest significance, it was found that locking a rigid cap to the cup or shell rim resulted in up to nine fewer impactions being necessary to seat it than impacting directly on the polar surface or using a cap free from the rim of the component, as is the case with many commercial resurfacing cup impaction devices currently used. This is important to impactor design and could make insertion easier and also reduce acetabula bone damage.     

An axisymmetric contact model of ultra high molecular weight polyethylene cups against metallic femoral heads for artificial hip joint replacements.

Contact mechanics of ultra high molecular weight polyethylene (UHMWPE) cups against metallic femoral heads for artificial hip joints is considered in this study. Both the experimental measurement of the contact area and the finite element prediction of the contact radius, maximum contact pressure and maximum Von Mises stress have been carried out for a wide range of contemporary artificial hip joints. Good agreement of the contact radius has been found between the experimental measurements and the finite element predictions based upon an elastic modulus of 1000 MPa and a Poisson's ratio of 0.4 for UHMWPE material under various loads up to 2.5 kN. It has been shown that the half contact angle for all the cup/head combinations considered in this study is between 40 degrees and 50 degrees under a load of 2.5 kN. The importance of this result has been discussed with respect to the anatomical position of the cup when placed in the body and the selection of a simple wear-screening test for artificial hip joints. The predicted contact radius and maximum contact pressure from the finite element model have also been compared with a simple elasticity analysis. It has been shown that the difference in the predicted contact radius between the two methods is reduced for more conforming contacts between the femoral head and the acetabular cup and smaller UHMWPE cup thickness. However, good agreement of the predicted maximum contact pressure has been found for all the combinations of the femoral head and the acetabular cup considered in this study. The importance of contact mechanics on the clinical performance of artificial hip joint replacements has also been discussed.     

A simple fully integrated contact-coupled wear prediction for ultra-high molecular weight polyethylene hip implants

A fully coupled contact and wear model was developed in the present study for hip implants employing an ultra-high molecular weight polyethylene (UHMWPE) cup in combination with a metallic or ceramic femoral head. A simple elasticity equation based on the concept of constrained column model was employed to solve the contact mechanics between the acetabular cup and the femoral head under the three-dimensional physiological loading condition. The wear model was based on the classical Archard-Lancaster equation in common with all other studies reported in the literature. The fully coupled contact and wear model was applied to both conventional and cross-linked UHMWPE cups under a wide range of design parameters such as the clearance and the femoral head radius. The predicted linear and volumetric wear as well as their rates for conventional UHMWPE cups were found to be in good agreement with those obtained from a similar analysis by Maxian but using the finite element method for the contact mechanics analysis. The predicted maximum contact pressure was found to decrease rapidly within the first 10(6) cycles, and below the limit to cause plastic deformation within the UHMWPE cup with a nominal radial clearance of 0.2 mm. The effect of the clearance between the head and the cup on the predicted wear was found to be negligible. For the cross-linked UHMWPE cup with relatively large diameters up to 48 mm and a fixed outside diameter of 50 mm, the predicted wear, which was found to increase with increasing femoral head radius, remained small owing to the small wear factor associated with these materials. Furthermore, if the head diameter increases beyond 42 mm, a rapid increase in the contact pressure was predicted, owing to the decrease in the wall thickness of the cross-linked UHMWPE cup.     

Elastohydrodynamic lubrication analysis of ultra-high molecular weight polyethylene hip joint replacements under squeeze-film motion

Elastohydrodynamic lubrication was analysed under squeeze-film or normal approach motion for artificial hip joint replacements consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup and a metallic or ceramic femoral head. A simple ball-in-socket configuration was adopted to represent the hip prosthesis for the lubrication analysis. Both the Reynolds equation and the elasticity equations were solved simultaneously for the lubricant film thickness and hydrodynamic pressure distribution as a function of the squeeze-film time was solved using the Newton-Raphson method. The elastic deformation of the UHMWPE cup was calculated by both the finite element method and a simple equation based upon the constrained column model. Good agreement of the predicted film thickness and pressure distribution was found between these two methods. A simple analytical method based upon the Grubin-Ertel-type approximation developed by Higginson in 1978 [1] was also applied to the present squeeze-film lubrication problem. The predicted squeeze-film thickness from this simple method was found to be remarkably close to that from the full numerical solution. The main design parameters were the femoral head radius, the radial clearance between the femoral head and the acetabular cup, and the thickness and elastic modulus for the UHMWPE cup; the effects of these parameters on the squeeze-film thickness generated in current hip prostheses were investigated.     

Contact mechanics analysis of metal-on-metal hip resurfacing prostheses

Finite-element method was employed to study the contact mechanics in metal-on-metal hip resurfacing prostheses, with particular reference to the effects of bone quality, the fixation condition between the acetabular cup and bone, and the clearance between the femoral head and the acetabular cup. Simple finite-element bone models were developed to simulate the contact between the articulating surfaces of the femoral head and the acetabular cup. The stresses within the bone structure were also studied. It was shown that a decrease in the clearance between the acetabular cup and femoral head had the largest effect on reducing the predicted contact-pressure distribution among all the factors considered in this study. It was found that as the clearance was reduced, the influence of the underlying materials, such as bone and cement, became increasingly important. Stress shielding was determined to occur in the bone tissue surrounding the hip resurfacing prosthesis considered in this study. However, the stress-shielding effects predicted were less than those observed in conventional total hip replacements. Both the effects of bone quality (reduction in elastic modulus) and the fixation condition between the cup and the bone were found to have a negligible effect on the predicted contact mechanics at the bearing surface. The loading was found to have a relatively small effect on the predicted maximum contact pressure at the bearing surface; this was attributed to an increase in contact area as the load was increased.     

A hip simulator study of metal-on-metal hip joint device using acetabular cups with different fixation surface conditions

In vitro wear data for hip joint devices reported in the literature vary in a wide range from one simulator study to another sometimes for the same type of device tested under identical physiological testing conditions. We hypothesized that non-bearing surface condition of the testing components could be an important factor affecting the simulator wear results. To confirm this hypothesis, fifteen 50 mm metal-on-metal hip resurfacing devices with identical bearing specifications were tested in a ProSim hip wear simulator for 5 million cycles. The heads were standard Birmingham Hip Resurfacing (BHR) heads; whilst the pairing acetabular cups were identical to the standard BHR cup except their different back surface conditions, including: (a) off-the-shelf products after removing the hydroxyapatite (HA) coating; (b) semi-finished products without HA coating; and (c) purposely-made cups without cast-in beads and HA coating. Results showed that the different back surfaces of the cups used indeed caused significantly large variations in the gravimetrically measured wear loss. We postulated that materials loss from the non-bearing surface of the testing components could contribute to the gravimetrically measured wear loss during a wear simulator test both directly and indirectly. The results presented in this paper pertain to In vitro wear simulator study and have little clinical relevance to the performance of any implant in vivo.     

Contact mechanics of a novel metal-on-metal total hip replacement

The contact mechanics of a novel metal-on-metal total hip replacement (THR) were investigated in this study. The metal-on-metal prosthesis considered consists of a cobalt-chrome acetabular insert connected to a titanium shell through a taper contact, articulating against a cobalt-chrome femoral head. Both the experimental measurement of the displacement of the acetabular insert and the contact area between the two bearing surfaces, and the corresponding numerical predictions using the finite element method have been conducted. Excellent agreement has been demonstrated between the experimental measurement and the finite element prediction under various loads up to 3 kN. The maximum contact pressure at the articulating surfaces has been predicted to be about 31 MPa from a simple axisymmetric finite element model, significantly lower than that of a similar cup but with a monoblock construct. This has been mainly attributed to the flexibility of the insert, leading to an increase in the conformity between the femoral head and the acetabular insert. In addition, the predicted maximum contact pressure is only slightly increased to 37 MPa, from a more realistic three-dimensional anatomical finite element model. The design features on metal-on-metal THRs have been shown to reduce contact stresses and may improve tribological performances of these hard-on-hard bearing couples.     

Volumetric wear assessment of failed metal-on-metal hip resurfacing prostheses

Recent advancements in hip arthroplasty have allowed the operation to boast excellent results and high survivorship. However, failures do still occur and a major cause is complications arising from wear debris. It is essential therefore that debris is minimized by reducing wear at the bearing surface. One proposed method of achieving this wear reduction is through the use of metal-on-metal articulations. One of the latest manifestations of this biomaterial combination is in designs of hip resurfacing which are aimed at younger, more active patients who might wear out a conventional metal-on-polymer hip prosthesis. However, do these metal-on-metal hip resurfacings show less wear when implanted into patients?Using a co-ordinate measuring machine and a bespoke computer program, volumetric wear measurements for retrieved Articular Surface Replacements (ASR™, DePuy) metal-on-metal hip resurfacings were undertaken. Thirty-two femoral heads and twenty-two acetabular cups were measured. Acetabular cups exhibited mean volumetric wear of 29.00 mm³ (range 1.35-109.72 mm³) and a wear rate of 11.02 mm³/year (range 0.30-63.59 mm³/year). Femoral heads exhibited mean wear of 22.41 mm³ (range 0.72-134.22 mm³) and a wear rate of 8.72 mm³/year (range 0.21-31.91 mm³/year). In the 22 cases where both head and cup from the same prosthesis were available, mean total wear rates of 21.66 mm³/year (range 0.51-95.50 mm³/year) were observed. Compared with in many vitro tests, these are significantly higher than those expected in a well functioning metal-on-metal hip resurfacing prosthesis and are of concern.     

Study of deformation and form error due to press-fitting a thick cylinder into a square outer housing

The deformation and form error resulting from press-fitting a thick cylinder into a square housing with an inner circular hole were studied using finite element analysis. The results show the effect of diametral interference and housing thickness on the resulting deformation and form error. The dimensions of an assembly of two thick cylinders equivalent to the assembly studied was determined.     

Hip resurfacing arthroplasty: the evolution of contemporary designs

Metal-on-metal hip resurfacing is considered by many as the most significant recent development in hip arthroplasty. It preserves proximal femoral bone stock, optimizes stress transfer to the proximal femur, and offers inherent stability and optimal range of movement. The early results of hip resurfacing in the 1970s and 1980s were poor and the procedure was largely abandoned by the mid-1980s. The expectation that these prostheses would be easy to revise was not often fulfilled. The large diameter of the articulation combined with thin polyethylene cups or liners resulted in accelerated wear and the production of large volumes of biologically active particulate debris, leading to bone loss and implant loosening. Failure has been attributed to other factors, mainly avascular necrosis of the femoral head. However, this concern has not been confirmed by retrieval studies. The failure of early hip resurfacings was essentially a consequence of the use of inappropriate materials, poor implant design, inadequate instrumentation, and crude surgical technique. It was not an inherent problem with the procedure itself. The renaissance of metal-on-metal articulations for total hip arthroplasty enabled the introduction of new hip resurfacings and most of the major implant manufacturers have already introduced such systems. Early results are encouraging and complications commonly seen in the 1970s and 1980s, such as early implant loosening and femoral neck fracture, now appear to be rare. Whilst early results should be regarded with caution, modern metal-on-metal hip resurfacing potentially offers the ultimate bone preservation and restoration of function in appropriately selected young patients.     

An elastohydrodynamic lubrication (EHL) model of wear particle migration in an artificial hip joint

A full fluid ball-in-socket elastohydrodynamic lubrication (EHL) analysis of an artificial hip joint made of a metallic femoral head and ultra-high molecular weight polyethylene (UHMWPE) acetabular cup was considered. Since artificial hips operate in a mixed lubrication mode, wear occurs and wear particles lead to reduced hip lifetimes. This study involves simulating these particles within the lubrication regime. Hip deformation was compared to models employing finite element analysis and the spherical fast-Fourier transform technique. Particle modeling results were compared to suspension modeling experiments by other researchers. Results show a strong influence of lubricant fluid velocity on that of the wear particles.     

Effects of acetabular resurfacing component material and fixation on the strain distribution in the pelvis

A 3D finite element (FE) model of an implanted pelvis was developed as part of a project investigating an all-polymer hip resurfacing design. The model was used to compare this novel design with a metal-on-metal design in current use and a metal-on-polymer design typical of early resurfacing implants. The model included forces representing the actions of 22 muscles as well as variable cancellous bone stiffness and variable cortical shell thickness. The hip joint reaction force was applied via contact modelled between the femoral and acetabular components of the resurfacing prosthesis. Five load cases representing time points through the gait cycle were analysed. The effect of varying fixation conditions was also investigated. The highest cancellous bone strain levels were found at mid-stance, not heel-strike. Remote from the acetabulum there was little effect of prosthesis material and fixation upon the von Mises stresses and maximum principal strains. Implant material appeared to have little effect upon cancellous bone strain failure with both bended and unbonded bone-implant interfaces. The unbonded implants increased stresses in the subchondral bone at the centre of the acetabulum and increased cancellous bone loading, resembling behaviour obtained previously for the intact acetabulum.     

A comparative joint simulator study of the wear of metal-on-metal and alternative material combinations in hip replacements

While total hip replacement represents the major success story in orthopaedic surgery in the twentieth century, there is much interest in extending even further, early in the twenty first century, the life of implants. Osteolysis has been identified as a major factor limiting the life of prostheses, with indications that fine polyethylene wear debris, generated primarily at the interface between the femoral head and the acetabular cup, promotes the process. There is therefore considerable interest in the introduction of alternative wear resistant systems to limit the deleterious effects of wear. These alternatives include ceramic-on-ceramic and metal-on-metal configurations and the present paper is primarily concerned with the latter. Some six pairs of new metal-on-metal implants of 36 mm diameter and four pairs of existing metal-on-metal implants of 28 mm diameter were tested in a ten-station hip joint simulator in the presence of a 25 per cent bovine serum solution. The implants were tested in the anatomical position to 5 x 10(6) cycles. The new heads and cups were manufactured from CoCrMo alloy with careful attention being paid to sphericity and surface finish of both components. The wear performance of the new and existing metal-on-metal total hip replacements have been evaluated and compared. The overall wear rates have then been compared with previously reported wear rates for a zirconia-on-polyethylene prosthesis of 22 mm diameter tested on the same simulator. The comparison is taken further by recalling published penetration data for metal-on-polyethylene implants of 22 and 28 mm diameter and converting these to volumetric wear rates. It was found that the heads and cups in metal-on-metal joints wore by almost equal amounts and that the opposing surfaces converged to similar surface roughness as the testing time increased. Steady state wear rates were generally achieved after 1-2 x 10(6) cycles. The mean long-term wear rates for the metal-on-metal prostheses were very low, being 0.36 mm3/10(6) cycles and 0.45 mm3/10(6) cycles for the new implants of 36 mm diameter and established implants of 28 mm diameter respectively. These wear rates compare with 6.3 mm3/10(6) cycles for zirconia-on-ultra-high molecular weight polyethylene tested on the same simulator and representative clinical values for metal-on-polyethylene of 36 mm3/year for heads of 22 mm diameter and a reported range of 60-180 mm3/year for 28 mm heads. These values do not translate directly into numbers of particles, since the metallic debris from metal-on-metal joints is very fine. The number of metallic particles may exceed the number of polyethylene wear particles from an otherwise similar metal-on-polyethylene joint by a factor of 10(3). A detailed discussion of the size and morphology of wear debris and tissue reaction to various forms of debris is beyond the scope of this paper, but the biological response to polymeric, metallic and ceramic wear debris forms a major subject for further study. The present investigation nevertheless confirms the potential of carefully designed and manufactured metal-on-metal total replacement joints for the treatment of diseased and damaged hips.     

深海刚柔组合负压吸附装置的力学特性与吸附效果分析

柔性吸盘被广泛用于陆上气动负压系统中,但对于深海高压环境来说,柔性材料抵抗外界压力的能力较差,导致其过度变形,吸盘有效作用面积减少,吸附能力减弱甚至丧失。通过综合考虑柔性吸盘的良好密封特性及刚性吸盘有效作用面积不变的特性,设计了一种由柔性吸盘、刚性吸盘及卡箍组成的吸盘形式,通过有限元仿真分析了吸盘的密封性能和预紧力情况,确定了吸盘的尺寸和选型,并搭建了试验台,对吸盘的预紧力、密封性能和吸附力进行了试验。结果表明:吸盘在预紧力为390 N时完全变形,此时吸盘在外界环境压力11 MPa,吸盘内部压力1 MPa的情况下,15 min内吸盘内部压力没有发生变化,吸盘没有发生泄漏,且可以提供至少15000 N的吸附力。