Terry G. Richard

The Mechanical Behavior of a Solid Microsphere Filled Composite

T WAS THE intent of this investigation to produce a birefringent solid, suitable Ifor holographic and photoelastic analysis, which had variable mechanical properties [1]. For this purpose a mechanically and optically homogeneous, isotropic material was needed. A material of this nature was obtained by fabrication of a composite system composed of solid glass microspheres embedded in a polyester matrix. A series of tensile tests was performed to characterize the response of this composite and define the variation of the mechanical properties with changes in the volume fraction of glass. The results of these tests are compared to’ existing theories which predict the influence of reinforcement on the overall composite response. Four different theories for the prediction of the mechanical behavior of particulate filled composites were studied. Each theory is employed to produce an analytical definition of the variation of the elastic modulus and Poisson’s ratio for this material as a function of glass content.

Dynamic biomechanical model of the hand and arm in pistol grip power handtool usage

The study considers the dynamic nature of the human power handtool operator as a single degree-of-freedom mechanical torsional system. The hand and arm are, therefore, represented as a single mass, spring and damper. The values of these mechanical elements are dependent on the posture used and operator. The apparatus used to quantify these elements measured the free vibration frequency and amplitude decay of a known system due to the external loading of the hand and arm. Twenty-five subjects participated in the investigation. A full factorial experiment tested the effects on the three passive elements in the model when operators exerted maximum effort for gender, horizontal distance (30, 60, 90 cm), and vertical distance (55, 93, 142 190 cm) from the ankles to the handle. The results show that the spring element stiffness and mass moment of inertia changed by 20.6 and 44.5% respectively with vertical location (p < 0.01), and 23.6 and 41.2% respectively with horizontal location (p < 0.01). Mass moment of inertia and viscous damping for males were 31.1 and 38.5% respectively greater than for females (p < 0.01). Tool handle displacement and hand force during torque buildup can, therefore, be predicted based on this model for different tool and workplace parameters. The biomechanical model was validated by recalling five subjects and having them operate a power handtool for varying horizontal distances (30, 60, 90 cm), vertical distances (55, 93, 142 cm), and two torque build-up times (70, 200 ms). Tool reaction displacement was measured using a 3D-motion analysis system. The predictions were closely correlated with these measurements (R = 0.88), although the model underpredicted the response by 27%. This was anticipated since it was unlikely that operators used maximal exertions for operating the tools.

A single-degree-of-freedom dynamic model predicts the range of human responses to impulsive forces produced by power hand tools.

The human operator is modelled as a single-degree-of-freedom dynamic mechanical system for predicting the response to impulsive torque reaction forces produced by rotating spindle power hand tools such as nutrunners or screwdrivers. The model uses mass, spring and damping elements to represent the standing operator supporting the tool in the hand. It was hypothesized that these mechanical elements are affected by work location and vary among individuals. These elements were ascertained by measuring the resulting frequency and amplitude of a freely oscillating defined mechanical system when externally loaded using maximal effort to oppose its motion. Twenty-five subjects (13 female, 12 male) participated in the full factorial experiment that measured the effects of gender, vertical and horizontal work location for various tool shapes (in-line, pistol, right angle), and orientations (horizontal and vertical). The mean operator stiffness decreased from 1721 to 1195 N/m when the horizontal work location increased from 30 to 90 cm in front of the ankles for a pistol-grip handle used on a vertical surface. Males had greater mass moment of inertia of (0.0099 kg m2) than females (0.0072 kg m2) for an in-line handle used on a horizontal surface. Internal validation by independently measuring apparatus torque found that the model satisfactorily explained the measured operator dynamics with an average error of 2.86%. Group variance reflects the range of operator capacities to react against power hand tool generated forces for the sample group and therefore it may also be useful for understanding the range of capacities among a group of operators performing similar tasks.

Forces associated with pneumatic power screwdriver operation: statics and dynamics

The statics and dynamics of pneumatic power screwdriver operation were investigated in the context of predicting forces acting against the human operator. A static force model is described in the paper, based on tool geometry, mass, orientation in space, feed force, torque build up, and stall torque. Three common power hand tool shapes are considered, including pistol grip, right angle, and in-line. The static model estimates handle force needed to support a power nutrunner when it acts against the tightened fastener with a constant torque. A system of equations for static force and moment equilibrium conditions are established, and the resultant handle force (resolved in orthogonal directions) is calculated in matrix form. A dynamic model is formulated to describe pneumatic motor torque build-up characteristics dependent on threaded fastener joint hardness. Six pneumatic tools were tested to validate the deterministic model. The average torque prediction error was 6.6% (SD = 5.4%) and the average handle force prediction error was 6.7% (SD = 6.4%) for a medium-soft threaded fastener joint. The average torque prediction error was 5.2% (SD = 5.3%) and the average handle force prediction error was 3.6% (SD = 3.2%) for a hard threaded fastener joint. Use of these equations for estimating handle forces based on passive mechanical elements representing the human operator is also described. These models together should be useful for considering tool handle force in the selection and design of power screwdrivers, particularly for minimizing handle forces in the prevention of injuries and work related musculoskeletal disorders.

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.

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.

Cellular concrete - A potential load-bearing insulation for cryogenic applications?

The need for low cost, low thermal conductivity, high strength insulation suitable for cryogenic applications is becoming more evident. An investigation of the potential of cellular concretes to fulfill this function has been initiated. A review of the thermal and mechanical characteristics of foamed plastics and cellular concrete is presented along with relative cost comparisons. Test data from preliminary investigations is presented to define the influence of material constituents, density and temperature on the mechanical and thermal response of cellular concrete. Specimen densities range from 0.64 to 1.44 gr/cc. The influence of temperature variations from 22°C to -196°C is reported for selected densities.

A Dynamic Biomechanical Model of the Hand and Arm in Pistol Grip Power Hand Tool Use

Torque build-up during power hand tool operation constitutes a relatively short period of time, but its influence on muscle exertion is most significant due to repeated exposure to relatively high reaction forces. This study considers the dynamic nature of the human power hand tool operator as a single degree of freedom mechanical system. The hand is therefore represented as mass, spring and damping elements. These are dependent upon the posture used and the individual operator. An apparatus was used to obtain these elements by measuring the vibration frequency and amplitude change after an initial disturbance. Twenty-five subjects participated in the experiment. A repeated measures full factorial experiment tested the effects of gender, horizontal distance (30, 60, and 90 cm), and vertical distance (55, 93, 142, and 190 cm) on the three passive elements in the model for a pistol grip handle. The result shows that stiffness and mass moment of inertia changed by 20.6% and 44.5% respectively with vertical location (p<.01), and 23.6% and 41.2% respectively with horizontal location (p<.01). Mass moment of inertia and damping changed by 31.1% and 38.5% respectively with gender (p<.01). The handle displacement and hand force during torque build-up was calculated based on this model. The model was validated by having five subjects operate an actual power hand tool and by measuring tool displacement using an OptoTrak motion analysis system. The model predictions were correlated with these measurements (R=.88).

Development and Validation of a Dynamic Biomechanical Model for Power Hand Tool Torque Build-up Reaction Force

This study models the human power hand tool operator as a dynamic single degree of freedom mechanical system for predicting the response to torque reaction forces produced by rotating spindle tools such as nutrunners or drills. The model represents the hand as mass, spring and damping elements that are dependent upon the posture used and the individual operator. It therefore considers tool shape (i.e., in-line, pistol, right angle), tool orientation (i.e., horizontal or vertical), horizontal location from the ankles, and vertical location from the ankles, in addition to gender. The apparatus used to quantify these elements measured the vibration frequency and amplitude change after an initial disturbance to a defined mechanical system. Twenty-five subjects (13 female, 12 male) participated in the experiment. A repeated measures design tested the effects of gender, work location, tool shape, and torque direction on the three passive elements in the model. The results show that stiffness for pistol grip and right angle handle used on a horizontal surface changed by 12% (p<.01) and inertial mass parameter changed by 29% (p<.001). Horizontal location was a significant factor for stiffness and inertial mass parameter for all four handle conditions.