Discomfort from pneumatic tool torque reaction: Acceptability limits

Pneumatic shut-off nut runners may produce large reaction forces to the operator's hand, especially at the end of the securing of threaded fasteners. These reaction forces depend on the shut-off mechanism, the joint hardness, tool torque level and to some extent the physical properties of the tool. The objective of this study was to find acceptability limits for the discomfort from pneumatic tool torque reaction forces that could be related to technical test measures of the tools.

In a study at a truck assembly industry 38 workers participated. Reaction forces, tool handle displacements and subjective discomfort ratings were measured. The tools were first tested according to ISO 6544 in the laboratory and the tool torque impulse was calculated. Strong correlations between tool handle displacements (r = 0.952), reaction forces (r = 0.981, vertical force) and ratings were found. Acceptability limits for ratings, tool handle displacements and reaction forces were also determined. No subject would accept to work a whole workday at a discomfort level over 9 on a 20-point scale and all would work a whole workday at a discomfort level of 2. These limits could then be correlated to the tool torque impulse measures from the technical test, thus making it possible to predict how many of the assemblers would work within acceptance limits.     

Pneumatic tool torque reaction: reaction forces, displacement, muscle activity and discomfort in the hand-arm system

Reaction forces, hand-arm displacement, muscle activity and discomfort ratings were studied during the securing of threaded fasteners with three angle nutrunners with different shut-off mechanisms, but with the same spindle torque (72-74 Nm). The three tools were tested according to the method specified in ISO 6544. One of the tools had an almost instantaneous shut-off. Another had a more slowly declining torque curve. For the third tool the maximum torque was maintained for a while before shut-off. Twelve male subjects participated in the study. A force platform measured the reaction force between the subject and the floor. The option of the hand-arm system and the shoulder was measured with an optoelectronic measuring system. The muscle activity (EMG) in six muscles in the arm and shoulder was measured with surface electrodes. Significant differences in the arm movements and ground reaction forces were found between the three tools. The smallest values were found with the fast shut-off tool while the delayed shut-off tool caused the largest values. The EMG measures gave inconsistent response patterns. Discomfort ratings were highly correlated with the time for which the tool torque exceeded 90% of peak preset torque, but the time for which the tool torque exceeded 90% of peak calculated by the method specified in ISO 6544. Nutrunners with a shut-off mechanism that causes a slowly decreasing torque or a torque that is maintained for a while before shut-off should be avoided. If no substitutes are available, then a torque reaction bar should be mounted on the tool.     

Handle displacement and operator responses to pneumatic nutrunner torque buildup

The objective of this study is to investigate the workstation and tool effects on the responses of the powered hand tool operator reacting against the impulsive reaction forces that may be associated with upper extremity musculoskeletal disorders. The study demonstrated a means of direct measurement of force at the interface between the tool and the operator. Fifteen experienced male operators performed three independent work configurations: pistol grip and right angle tools used on the horizontal surface and pistol grip tools used on the vertical surface, in the laboratory. A full factorial experiment consisting of 36 conditions was designed to examine the effects of working height, distance, tool, and fastener joint hardness on handle displacement and grip forces. The results indicate that operator responses were affected by different factors depending on the work configuration. When pistol grip tools were used on the vertical surface, the mean handle displacement decreased from 9.9 degrees to 7.3 degrees as the working height increased from 30 cm below shoulder to 30 cm above shoulder. When right angle tools were used, the greatest handle displacement (51.1mm) and grip force (84.7% MVC) during torque reactions were measured at 30 cm below elbow and 40% forward reach away from the operator. This study provides quantitative information that can be used for workstation design and tool selection to reduce the torque reaction experienced by powered nutrunner operators.     

A system for the measurement of grip forces and applied moments during hand tool use

Quantification of the forces applied with or by hand tools can be a difficult but important component of an ergonomic evaluation. This paper describes a device for measuring gripping forces and the moments generated by a hand tool. Laboratory characterization indicated that the device had good linearity (r2 = 0.999) with minimal hysteresis or creep. The working range exceeds 700N for gripping forces, and 28 and 16Nm for the two applied moment axes. The device, configured as a boning knife, was sensitive to differences in grip forces and applied moments in a simulated meat cutting task requiring distinct levels of precision. Significant individual variation in the "efficiency" of grip force was also observed. The system design is flexible, allowing for additional tool configurations.     

Muscle responses to simulated torque reactions of hand-held power tools

The aim of this work was to investigate physiological responses to torque reaction forces produced by hand-held power tools used to tighten threaded fasteners. Such tools are used repetitively by workers in many industries and are often associated with upper limb musculoskeletal complaints. The tools considered for stimulation in this study had straight handles and required from 100 to 400 ms to tighten fasteners to a peak torque of 1.0 to 2.5 Nm and from 50 to 150 ms for the torque to decay to zero. A tool stimulator was constructed to apply a programmed torque profile to a handle similar to that of a straight in-line power screwdriver. Wrist flexor and extensor surface EMGs and handle position were recorded as subjects held handles subjected to controlled torque loads that tended to flex the wrist. It was found that: (1) very high EMG values occurred even though torques were of short duration (50 to 600 ms) and the peak torques were low (7–28% of maximum strength); (2) high EMGs in anticipation of torque are directly related to torque build-up rate and peak torque; (3) high peak flexor and extensor EMGs during and following torque onset are related to torque build-up rate and peak torque; (4) minimum time of peak EMGs of 72–87 ms following the onset of torques with 50 ms build-up suggests the contribution of an extensor muscle stretch reflex component; delayed peak for longer build-ups suggests a central control of muscle force in response to torque; (5) angular excursions of handles increase with decreasing torque build-up time and increasing torque magnitude causes increasing eccentric work; (6) the results show that the slow torque build-up times (450 ms) correspond to minimum peak EMGs; and (7) accumulated EMGs increase with increasing torque and torque build-up times. Further studies are needed to evaluate fatigue and musculoskeletal injuries associated with prolonged periods of tool use.     

The effects of joint torque, pace and work:rest ratio on powered hand tool operations

Repetitive use of hand-held power tools is associated with work-related upper extremity musculoskeletal disorders. Using a pneumatic nutrunner, 21 men completed twelve 360 repetitive fastener-driving sessions on three joints (hard, soft and control) at slow and fast pace, and two different work:rest patterns. Handgrip force and perceived exertions were collected throughout each session. For the control joint, the mean grip force exerted was 39.6% of maximum voluntary exertion (MVE) whereas during hard and soft joint sessions it was 48.9% MVE and 56.9% MVE, respectively. Throughout each session, the grip force decreased, more while operating soft and hard joints as compared with the control joint (regression slope: ?0.022 and ?0.023, compared with ?0.007 N/drive, respectively), suggesting considerable upper extremity muscular effort by participants during torque buildup. Fast work pace resulted in higher average grip forces by participants but a greater decrease in the force as the session progressed. Providing rest breaks reduced perceived exertions. The findings gain additional knowledge for assembly task design to possibly reduce the hand/arm injury risks for the operator.

Practitioner Summary: Powered hand tools are widely used in assembly and manufacturing industries. However, the nature of their repetitive use on human operator biomechanical and perceptual responses is not fully understood. This study examined work-related risk factors such as joint torque, pace and work:rest ratios on powered hand tool performance.     

Design and development of a pneumatic anthropomorphic hand

This work deals with a pneumatically actuated hand composed of four fingers and an opposable thumb. Mechanisms to convert actuator motion into phalanx rotation were studied to make each finger as similar as possible to the human specimen. Force tactile sensors are disposed along the phalanxes to allow a closed‐loop force control, while the thumb position is sensorized by a potentiometer and hence position controlled. Each component is controlled by fuzzy logic. This solution allowed the existing strong nonlinearities to be easily managed. A fuzzy supervisor applies a grasping strategy whose target is an approximate identification of the shape and size of an object to grasp it most efficiently with the disposable fingers. ©2000 John Wiley & Sons, Inc.     

多指灵巧手的位置/力矩控制

提出了一种新的多指手位置/力矩控制策略.该策略基于滑模位置控制和PD力矩控制分别实现自由空间中的轨迹跟踪和约束空间中的力矩跟踪,利用简单的系统观测器实现灵巧手在过渡过程中的控制模式切换.该策略具有很好的位置/力矩跟踪能力,并且可以保证过渡过程的稳定性.实验结果证明了该控制策略的有效性和可行性.     

Coordination control of an active pneumatic deburring tool

An efficient robotic deburring method was developed based on a new active pneumatic tool. The developed method considers the interaction among the tool, the manipulator and the workpiece and couples the tool dynamics and a control design that explicitly considers deburring process information. The new active pneumatic tool was developed based on a single pneumatic actuator with a passive chamber to provide compliance and reduce the chatter caused by air compressibility. A coordination control method was developed for efficient control of the system, which adopts two-level hierarchical control structure based on a coordination scheme. Robust feedback linearization was utilized to minimize the undesirable effect of external disturbances such as static and Coulomb friction and nonlinear compliance of the pneumatic cylinder stemming from the compressibility of air. The developed coordination control method demonstrated its efficacy in terms of deburring accuracy and speed.     

Predicting subjective perceptions of powered tool torque reactions

Powered hand tools have the potential to produce reaction forces that may be associated with upper extremity musculoskeletal disorders. In this study, subjective ratings of discomfort and acceptability of reaction forces were collected in an attempt to identify their associations with factors such as work location, and response covariates such as grip force and tool handle displacement. Three work configurations using pistol grip and right angle pneumatic nutrunners on horizontal and vertical surfaces were set up in the laboratory. Twenty healthy right-handed male participants operated four tools at nine locations and the corresponding subjective responses were collected. The results indicate that normalized grip force during the torque buildup period was a significant factor for both subjective ratings. For the unacceptable torque reactions across the three tool configurations, the ratio of hand moment impulse over tool torque impulse was significantly greater than for the acceptable reactions. For pistol grip tools used on the vertical surface, as the working height increased 30 cm, the odds of an unacceptable rating over an acceptable rating increased 1.6 times. Prediction models for subjective ratings of discomfort and acceptability provide insight regarding either workstation design or exposure control. These models can further be used to establish exposure limits based on handle displacement and grip force.     

Short-term changes in upper extremity dynamic mechanical response parameters following power hand tool use

Dynamic mechanical response parameters (stiffness, damping and effective mass), physiological properties (strength and swelling) and symptoms of the upper limb were measured before power tool operation, immediately following and 24?h after power tool operation. Tool factors, including peak torque (3 Nm and 9 Nm) and torque build-up time (50?ms and 250?ms), were controlled in a full factorial design. Twenty-nine inexperienced power hand tool users were randomly assigned to one of four conditions and operated a pistol grip nutrunner four times per min for 1?h in the laboratory. Isometric strength decreased immediately following tool use (15%) (p?<?0.01) and 24?h later (9%) (p?<?0.05). Mechanical parameters of stiffness (p?<?0.05) and effective mass (p?<?0.05) were affected by build-up time. An average decrease in stiffness (43%) and effective mass (57%) of the upper limb was observed immediately following pistol grip nutrunner operation for the long (250?ms) build-up time. A previously developed biomechanical model was used to estimate handle force and displacement associated with the tool factors in the experiment. The conditions associated with the greatest predicted handle force and displacement had the greatest decrease in mechanical stiffness and effective mass, and the greatest increase in localized discomfort.     

Short-term changes in upper extremity dynamic mechanical response parameters following power hand tool use

Dynamic mechanical response parameters (stiffness, damping and effective mass), physiological properties (strength and swelling) and symptoms of the upper limb were measured before power tool operation, immediately following and 24 h after power tool operation. Tool factors, including peak torque (3 Nm and 9 Nm) and torque build-up time (50 ms and 250 ms), were controlled in a full factorial design. Twenty-nine inexperienced power hand tool users were randomly assigned to one of four conditions and operated a pistol grip nutrunner four times per min for 1 h in the laboratory. Isometric strength decreased immediately following tool use (15%) (p < 0.01) and 24 h later (9%) (p < 0.05). Mechanical parameters of stiffness (p < 0.05) and effective mass (p < 0.05) were affected by build-up time. An average decrease in stiffness (43%) and effective mass (57%) of the upper limb was observed immediately following pistol grip nutrunner operation for the long (250 ms) build-up time. A previously developed biomechanical model was used to estimate handle force and displacement associated with the tool factors in the experiment. The conditions associated with the greatest predicted handle force and displacement had the greatest decrease in mechanical stiffness and effective mass, and the greatest increase in localized discomfort.     

An ergonomic comparison of pneumatic and electrical pistol grip hand tools

The purpose of this study was to compare the ergonomic demands associated with air and DC pistol grip hand tool use. Seven channels of EMG data were collected from 15 male and 15 female subjects to estimate the muscular demands on the forearms, biceps, shoulders and neck. An accelerometer was also used to estimate the torque reaction transmitted to the hand. Subjects performed drilling with five pistol grip tools obtained from two tool suppliers. This resulted in two air tools, one non-transducerized electric (DC_NT) and two transducerized electric (DC_T) tools. Three types of joints were simulated: (1) , (2) and (3) . Subjects were asked to drill five joints to completion within a period, and to repeat this five times for each condition (25 joints per condition). Results of a three factor repeated measures ANOVA indicated that, in comparison to air tools, pistol grip DC tools do not pose an additional risk of musculoskeletal injury to the upper limbs. In fact, the data suggest that the use of pistol grip DC tools will reduce the demands on the forearms during horizontal drilling.

Relevance to industry

Powered hand tools can potentially cause upper limb injuries due to the postures, repetitions, and forces associated with their use. However, while DC tools have engineering advantages (loop monitoring of the tool, and increased quality control) the ability to implement ergonomic strategies also appears to reduce the muscle demands associated with tool use.     

Muscle torque generators in multibody dynamic simulations of optimal sports performance

Multibody System Dynamics - Using detailed musculoskeletal models in computer simulations of human movement can provide insights into individual muscle and joint loading; however, these muscle...     

Power hand tool vibration effects on grip exertions

Operation of vibrating power hand tools can result in excessive grip force, which may increase the risk of cumulative trauma disorders in the upper extremities. An experiment was performed to study grip force exerted by 14 subjects operating a simulated hand tool vibrating at 9.8 m/s² and 49 m/s² acceleration magnitudes, at 40 Hz and 160 Hz frequencies, with vibration delivered in three orthogonal directions, and with 1.5kg and 3.0kg load weights. Average grip force increased from 25.3 N without vibration to 32.1 N (27%) for vibration at 40 Hz, and to 27.1N (7%) for vibration at 160 Hz. Average grip force also increased from 27.4 N at 9.8 m/s² acceleration to 31.8 N (16%) at 49m/s². Significant interactions between acceleration x frequency, and frequency x direction were also found. The largest average grip force increase was from 25.3N without vibration to 35.8N (42%) for 40 Hz and 49 m/s² vibration. The magnitude of this increase was of the same order as for a two-fold increase in load weight, where average grip force increased from 22.5N to 35.0N (56%). A second experiment studied hand flexor and extensor muscle responses using electromyography for five subjects holding a handle vibrating at 8 m/s² using ISO weighted acceleration, with frequencies of 20 Hz, 40 Hz, 80 Hz and 160 Hz, and grip forces of 5%, 10% and 15% of maximum voluntary contraction. Muscle responses were greatest at frequencies where grip force was affected, indicating that the tonic vibration reflex was the likely cause of increased grip exertions.     

EMG control of a pneumatic 5-fingered hand using a Petri net

This article presents the control method for a 5-fingered artificial hand using electromyography (EMG) signals. Our targeted artificial hand is driven by pneumatic actuators to reduce its weight, and we use ON/OFF solenoid valves instead of electro pneumatic regulators to simplify the control system. The pneumatic hand has 15 degrees of freedom, and it seems difficult to reproduce all the finger motions from the EMG signals only. Therefore, we describe typical hand motions using a Petri net, and control the finger motions efficiently based on this model. Each state of the Petri net indicates one step in the hand posture to complete the intended motion. Simultaneously, this state corresponds to the ON/OFF pattern of the 15 solenoid valves. This enables the operator to control the 5-fingered dexterous hand smoothly, transiting the state in the Petri net according to the EMG motion signals. We conducted an experiment to verify the validity of the proposed method. In the experiment, five typical motions (spherical grasp, power grip, hook grip, key grip, precision grip) were successfully performed using the 6-channel EMG signals measured from the operator’s forearm.     

Handle dynamics predictions for selected power hand tool applications

This study uses a previously developed single-degree-of-freedom mechanical model to predict the power hand tool operator handle kinematic response to impulsive reaction forces (Lin, 2001). The model considers the human operator as a lumped parameter passive mechanical system, consisting of stiffness, mass moment of inertia, and viscous damping elements. Six power nutrunners were operated by 9 volunteers (3 men, 6 women) in the laboratory, and corresponding handle kinematics were compared against model predictions. A full-factorial experiment considered torque buildup time and work location. Normalized forearm flexor EMG was measured to quantify muscle exertions and used to proportionally adjust the stiffness parameter. The measured handle displacement for actual tool operation strongly correlated to the model predictions (R = .98) for all handle configurations. The overall model prediction error was 3% for predicting tool handle responses to impulsive reaction forces for various tool and workstation parameters. This model should make it possible for designers to identify conditions that minimize the torque reaction experienced by power hand tool operators.     

Effect of body posture and common hand tools on peak torque exertion capabilities

An experimental investigation was conducted to investigate the effects of body posture and of different types of common non-powered hand tools on maximum volitional torque exertion capabilities of males and females. Thirty-six males and 14 females applied peak torque in 21 different body postures while using nine different hand tools. The data analysis indicated that, for both males and females, the magnitude of volitional torque is strongly influenced by the type of tool used and the posture assumed. Between the two, however, the effect of hand tool is more profound. The correlation between the tool type and volitional torque was of the order of 0.71. No other variable was as strongly correlated with volitional torque. Maximum volitional torque profiles, as a function of posture and tool type, are provided for males and females.