Required Coefficient Of Friction Research Articles

The effects of an upper limb exoskeleton on gait performance and stability

Upper limb exoskeletons (ULEs) are emerging as workplace tools to alleviate workload and prevent work-related musculoskeletal disorders during lifting tasks. However, their introduction raises concerns about potential instability and increased fall risk for workers. This study investigates gait performance and stability parameters implications of ULE use. Fifteen participants performed a carrying task with different loads (0, 5, 10, 15 kg), both with and without the use of an ULE. Spatiotemporal gait parameters, Required Coefficient of Friction (RCoF), Minimum Foot Clearance (MFC), and Margin of Stability (MoS) were analysed. The findings indicate that while the ULE does not significantly alter most gait parameters or slip risk, it may negatively impact trip risk. Furthermore, while mediolateral stability remains unaffected, anteroposterior stability is compromised by ULE usage. These insights are critical for ensuring the safe implementation of ULEs in occupational settings.

Surface Design of 3D-printed Polyether Ether Ketone for Abrasive Wear Resistance

The surface features of additively manufactured polymer parts differ compared to extruded polymer parts and decide their performance in terms of power transmission applications. Layer thickness is one of the important process parameters chosen based on the part complexity and production speed, influencing the surface features. The abrasive wear performance of high-performance polyether ether ketone (PEEK), which determines the performance of power transmission elements such as bearings and gears, is reported in this article. The layer thickness employed in fabrication, which varies from 0.1 mm to 0.35 mm, influences the surface roughness, real contact area, and hardness of the as-printed PEEK. The friction coefficient and wear resistance of a printed part are influenced by the layer thickness employed, indicating the need for a proper choice of layer thickness for power transmission applications. Samples printed with a layer height of 0.25 mm and higher exhibited an increase of friction coefficient and wear loss due to the weak surface integrity. The friction and wear characteristics of additive manufactured samples are compared with extruded samples. Wear debris accommodation in the grooves of the as-printed surfaces accelerates the wear due to three-body abrasion in high layer thickness samples. The micromechanisms of wear damage in different layer thickness samples are discussed. The observations indicate the strong influence of layer thickness and facilitate the selection of appropriate layer thickness for the required friction coefficient in a particular application.

Correlation between Postural Stability and Lower Extremity Joint Reaction Forces in Young Adults during Incline and Decline Walking

Postural stability may be affected during slope walking, as there are different body kinetics and kinematic responses compared with level walking. Understanding body adaptations toward different inclinations is essential to prevent the risk of injury from falls or slips. This study was conducted to determine the correlations between stability parameters and loading response in terms of joint reaction force at the lower-extremity joints during inclined and declined walking. Twenty male subjects walked in the level, incline, and decline directions on a custom-built platform at three different slope angles (i.e., 5°, 7.5°, and 10°). To determine the ground reaction force (GRF), joint reaction force (JRF), center of pressure (COP), and center of mass (COM), a motion capture system was used to read the data of the ten reflective markers and transfer them to visual three-dimensional (3D) software. Pearson’s correlation test was performed with statistical significance set at p < 0.05 to evaluate the correlation of the required coefficient of friction (RCOF), postural stability index (PSI), and COP-COM distance with the JRF. This study has identified that the JRF changes in opposition to the changes in the RCOF during the initial strike during incline and decline walking, as JRF increases, the RCOF decreases with different strengths of correlation. There is also a strong positive correlation between the PSI and JRF in the proximal–distal direction, where the JRFs change in accordance with the change in the PSI, and the JRF increases with the increment of PSI. In addition, the JRF of the lower extremity also changed in a manner similar to the COP-COM distance in the medial–lateral direction. Overall, each stability parameter was correlated with the JRF of the lower-extremity joints in different directions and strengths. This study demonstrated that slope walking is particularly affected by surface inclination in terms of stability and loading. Therefore, this research can serve as a basis for future studies on slopes, as there is no specific basis for a maximum degree of inclination that is safe and suitable for all applications.

During roof-to ladder transitions, walk-through extensions modify required friction direction

Severe and fatal falls involving ladders commonly occur during transitions across the ladder and another support surface. Slipping is a common initiating event in ladder falls. This study characterized the friction requirements and body kinematics of descending roof-to-ladder transitions with and without a walk-through extension. Healthy adults who regularly climb ladders (n = 17) completed descending roof-to-ladder transitions, while foot-rung kinetics and body kinematics were recorded. The peak required coefficient of friction (RCOF) with respect to the plane of the shoe sole was calculated. The RCOF and body angle were calculated using their resultant values and projections in the frontal and sagittal planes. Foot angle was calculated in the sagittal plane. Repeated-measures ANOVA determined that compared to a walk-through ladder, a traditional ladder was associated with a higher RCOF in the medial–lateral (ML) direction (F1,16 = 190.07, p < 0.001) and a lower RCOF in the anterior-posterior (AP) direction (F1,16 = 11.02, p = 0.004), but had no significant relationship with the resultant RCOF (F1,16 = 0.098, p = 0.76). Spearman’s rho tests performed across all testing configurations identified significant associations between foot angle and overall RCOF (rs = −0.724, p < 0.001), foot angle and AP RCOF (rs = −0.871, p < 0.001), and frontal plane body angle and ML RCOF (rs = 0.782, p < 0.001). Clustering in the data suggests that ladder attachments reduced frontal plane kinematics, which altered the direction of RCOF by reducing the medial–lateral component. These results have implications for designing rungs with good friction in multiple directions and the potential for body position monitoring in ladder tasks.

A parametric study of low friction coating by PVD method on the spiralock female thread for ITER application

The hard lubrication is defined that the thin films are deposited by radio-frequency magnetron sputtering from the target material on substrate in order to improve their desired properties such as hardness, friction and wear resistance. Especially, the MoS2 coating can be used in high vacuum and aerospace environments for lubrication purposes, which low friction is required. Unlike fluid lubrications, MoS2 is suitable for use in a vacuum environment because it has low outgassing pressure and lack of migration. For this reason, the sputtered MoS2 coating is primarily considered as low friction / anti-seize coating method of the ITER blanket central bolt insert. A magnetron sputtering process, which permits friction coefficient and durability of the MoS2 coating, suites to coat precision mechanical component with its exquisitely thin thickness. However, the coating on the spiralock female thread of blanket central bolt inserts is difficult to have uniform coating deposition due to its shadow effect. Therefore, it is necessary for understanding the parameters and effects of sputtering process to achieve the uniform coating having the required friction coefficient less than 0.05. Details of parametric study with various sputtering conditions on the simple disk and spiralock female thread to reach required friction coefficient and uniform coating thickness, are presented in this paper.

Quantification of Gait Stability During Incline and Decline Walking: The Responses of Required Coefficient of Friction and Dynamic Postural Index.

This study aims to investigate the gait stability response during incline and decline walking for various surface inclination angles in terms of the required coefficient of friction (RCOF), postural stability index (PSI), and center of pressure (COP)–center of mass (COM) distance. A customized platform with different surface inclinations (0°, 5°, 7.5°, and 10°) was designed. Twenty-three male volunteers participated by walking on an inclined platform for each inclination. The process was then repeated for declined platform as well. Qualysis motion capture system was used to capture and collect the trajectories motion of ten reflective markers that attached to the subjects before being exported to a visual three-dimensional (3D) software and executed in Matlab to obtain the RCOF, PSI, as well as dynamic PSI (DPSI) and COP-COM distance parameters. According to the result for incline walking, during initial contact, the RCOF was not affected to inclination. However, it was affected during peak ground reaction force (GRF) starting at 7.5° towards 10° for both walking conditions. The most affected PSI was found at anterior-posterior PSI (APSI) even as low as 5° inclination during both incline and decline walking. On the other hand, DPSI was not affected during both walking conditions. Furthermore, COP-COM distance was most affected during decline walking in anterior-posterior direction. The findings of this research indicate that in order to decrease the risk of falling and manage the inclination demand, a suitable walking strategy and improved safety measures should be applied during slope walking, particularly for decline and anterior-posterior orientations. This study also provides additional understanding on the best incline walking technique for secure and practical incline locomotion.

Shoe tread wear occurs primarily during early stance and precedes the peak required coefficient of friction

Worn shoes contribute to injuries caused by slip-and-fall accidents. The peak required coefficient of friction (RCOF) has been associated with tread wear rate. However, the temporal relationship between RCOF and shoe wear is unknown. The purpose of this study was to determine whether the contact region at the time of peak RCOF is consistent with the region of shoe wear. The shoe contact region at peak RCOF was imaged by frustrated total internal reflection. Images of worn tread after months of use were captured. The worn tread region was more posterior than the contact region at RCOF and did not correlate with the contact region at the time of RCOF. The contact regions observed during earlier stance (within 83 ms of heel contact) were more consistent with the worn region, suggesting that RCOF may not directly cause tread wear. These results serve to motivate future studies to identify early stance gait parameters associated with tread wear development.

Effects of back-support exoskeleton use on gait performance and stability during level walking

BackgroundBack-support exoskeletons (BSEs) are a promising intervention to mitigate physical demands at work. Although growing evidence indicates that BSEs can reduce low-back physical demands, there is limited understanding of potential unintended consequences of BSE use, including the risk of falls. Research questionDoes using a BSE adversely affect gait performance and stability, and are such effects dependent on specific BSE external torque characteristics? MethodsTwenty participants (10 M, 10 F) completed five level over-ground walking trials and a five-minute treadmill walking trial while wearing a BSE (backX™) with three different levels of external torque (i.e., no torque, low torque, and high torque) and in a control (no-exoskeleton) condition. Spatiotemporal gait patterns, stride-to-stride gait variability measures, required coefficient-of-friction (RCoF), and minimum foot clearance (MFC) were determined, to assess gait performance. Gait stability was quantified using the maximum Lyapunov exponent (MLE) of trunk kinematics and the margin-of-stability (MoS). ResultsUsing the backX™ with high supportive torque decreased slip risk (7% decrease in RCoF) and slightly improved trunk stability (3% decrease in MLE). However, it also decreased step length (1%), increased step width (10%) and increased gait variability (8–19%). Changes in MoS were complex: while MoS at heel strike decreased in the AP direction, it increased in the ML direction. There was a rather large decrease in MoS (26%) in the ML direction during the swing phase. SignificanceThis is the first study to quantify the effects of wearing a passive BSE with multiple supportive torque levels on gait performance and stability during level walking. Our results, showing that the external torque of the BSE may adversely affect gait step width, variability, and dynamic stability, can contribute to better design and practice guidelines to facilitate the safe adoption of BSEs in the workplace.

Performance optimization of a passively articulated mobile robot by minimizing maximum required friction coefficient on rough terrain driving

Though trainability (TA) and maneuverability (MA) are widely used for performance evaluations of mobile robots designed for rough terrain driving, we still have more challenging thing that has not yet been done before. That is experimental validation of the fact that the TA and MA can be simultaneously improved by design optimization which minimize the maximum required friction coefficient (MRFC) for the rough terrain driving robot. To this end, kinematics-based contact analysis and kineto-statics-based optimal torque distribution are performed to calculate RFC profile of a mobile robot on three-dimensional rough terrain. Based on these procedures, design optimization is carried out using MRFC as its cost function, and the results are validated through both Recurdyn simulation and field experiments. Two versions of the robot with initial and optimal dimensions are manufactured, to directly compare every result from the analysis. As results, it has been confirmed that the peak RFC was decreased by 21.8% from 0.174 to 0.136, TA was decreased by 10.7% from 10.3 Nm to 9.2 Nm, and MA was decreased by 26.7% from 29.2 mm to 21.4 mm. Also, the significance of this optimization strategy is verified under various conditions through Recurdyn simulations.

Gait kinetics impact shoe tread wear rate

BackgroundAdequate footwear is an important factor for reducing the risk of slipping; as shoe outsoles wear down, friction decreases, and slip and fall risk increases. Wear theory suggests that gait kinetics may influence rate of tread wear. Research questionDo the kinetics of walking (i.e., the shoe-floor force interactions) affect wear rate? MethodsFourteen participants completed dry walking trials during which ground reaction forces were recorded across different types of shoes. The peak normal force, shear force, and required coefficient of friction (RCOF) were calculated. Participants then wore alternating pairs of shoes in the workplace each month for up to 24 months. A pedometer was used to track the distance each pair of shoes was worn and tread loss was measured. The wear rate was calculated as the volumetric tread loss divided by the distance walked in the shoes. Three, mixed linear regression models were used to assess the impact of peak normal force, shear force, and RCOF on wear rate. ResultsWear rate was positively associated with peak RCOF and with peak shear force, but was not significantly related to peak normal forces. SignificanceThe finding that shear forces and particularly the peak RCOF are related to wear suggests that a person’s gait characteristics can influence wear. Therefore, individual gait kinetics may be used to predict wear rate based on the fatigue failure shoe wear mechanism.

Ricochets on asteroids: Experimental study of low velocity grazing impacts into granular media

Spin off events and impacts can eject boulders from an asteroid surface and rubble pile asteroids can accumulate from debris following a collision between large asteroids. These processes produce a population of gravitational bound objects in orbit that can impact an asteroid surface at low velocity and with a distribution of impact angles. We present laboratory experiments of low velocity spherical projectiles into a fine granular medium, sand. We delineate velocity and impact angles giving ricochets, those giving projectiles that roll-out from the impact crater and those that stop within their impact crater. With high speed camera images and fluorescent markers on the projectiles we track spin and projectile trajectories during impact. We find that the projectile only reaches a rolling without slipping condition well after the marble has reached peak penetration depth. The required friction coefficient during the penetration phase of impact is 4–5 times lower than that of the sand suggesting that the sand is fluidized near the projectile surface during penetration. We find that the critical grazing impact critical angle dividing ricochets from roll-outs, increases with increasing impact velocity. The critical angles for ricochet and for roll-out as a function of velocity can be matched by an empirical model during the rebound phase that balances a lift force against gravity. We estimate constraints on projectile radius, velocity and impact angle that would allow projectiles on asteroids to ricochet or roll away from impact, finally coming to rest distant from their initial impact sites.

Characterizing the shoe-rung friction requirements during ladder climbing

Ladder slip and falls cause negative financial and health impacts due to their severity and frequency. However, frictional requirements of climbing, which presumably influence slip risk, are unknown. The purpose of this study was to quantify frictional requirements during climbing at different ladder angles. The required coefficient of friction (RCOF) was calculated during ladder climbing and assessed for three ladder angles (75.5°, 82.8°, and 90°). Data was collected from 10 participants at each angle. Kinetic data and kinematic data of the climber’s shoe was collected and used to map friction and normal forces to the shoe. These forces were then used to calculate the RCOF. The RCOF and friction force at 90° was higher than at lower angles (75.5° and 82.8°). RCOF was correlated with shoe angle and body angle supporting both the relevance of shoe orientation and body position to slip risk. This study suggests that frictional requirements of ladder climbing are dependent on ladder angle, shoe orientation, and body angle.

Distribution of the local required coefficient of friction in the shoe–floor contact area during straight walking: A pilot study

We developed a shoe mounted with miniature tri-axial force sensors and investigated the distribution of the local required coefficient of friction (RCOF) in the contact area between the shoe sole and floor during straight walking. Four miniature force sensors were mounted on the outsole of the shoe. The study comprised five healthy young adult males (mean age: 22.4 years). Participants were instructed to walk straight at a normal pace on a level resin floor under 25 different sensor layouts to measure the three-directional ground reaction forces (GRFs) at 52 local positions in the region of the shoe sole. The local RCOF value (the maximum peak value of the ratio of the horizontal GRF to vertical GRF) was calculated at each sensor position. The mean local RCOF values at ten local positions were observed to be >0.4, whereas those at the lateral rear foot and toe were observed to be >0.6. These local RCOF values were much higher than the mean RCOF values calculated from the resultant local GRFs (0.18). The results of the present study will provide information on the distribution of friction requirements and the direction of applied horizontal GRF will aid in the development of slip-resistant shoe sole patterns.

Analysis of required coefficient of friction in running and walking

ABSTRACT The popularity of running has increased over the past few years. However, just a few studies in running have focused on the friction between surface and shoe/foot. Changes in friction can affect aspects of human motion, such as safety, motion pattern and efficiency among others. The aim was to investigate the effects of cadence (walk, self-selected running and imposed-running), stance sub-phases (absorption and propulsion) and footwear (barefoot and shod) on the required coefficient of friction (RCOF) of regular runners. Twenty healthy runners (12 males, 8 females, 29.4 ± 4.9 years, 70.4 ± 9.6 kg) participated in this study. Two force plates were used to measure the ground reaction forces (GRF) in order to calculate the RCOF for each condition and the stance phase was divided in sub-phases. In walk, the RCOF was smaller in the absorption than in propulsion phase (p < 0.001). Results evidenced effects of the cadence (p < 0.001), stance sub-phases (p < 0.001) and footwear (p < 0.001) on the RCOF. There was interaction effect in cadence with stance sub-phases (p < 0.001) and footwear with stance sub-phases (p < 0.001). Our results show RCOF is influenced by cadence and footwear condition in the absorption phase.

Effects of age-related changes in step length and step width on the required coefficient of friction during straight walking

BackgroundSlipping is one of the leading causes of falls among older adults. Older adults are considered to walk with a small anteroposterior (AP) component and a large mediolateral (ML) component of the required coefficient of friction (RCOF) owing to a short step length and a wide step width, respectively. However, limited information is available. Research questionWhat are the effects of aging on the resultant RCOF (RCOFres) and its ML (RCOFML) and AP (RCOFAP) components during straight walking? MethodsWe used the kinetic and kinematic data of 188 participants aged 20–77 years from a publicly available database (National Institute of Advanced Industrial Science and Technology Gait Database 2015). The participants were divided into the following three groups: young group (n = 56; age range, 20–34 years), middle-aged group (n = 50; age range, 35–64 years), and old group (n = 82; age range, 65–77 years). ResultsThe RCOFres and RCOFAP were lower in the old group than in the other groups, indicating a lower slip risk in this group. However, the RCOFML was higher and the step width was greater in the old group than in the other groups. The higher RCOFML and lower RCOFAP in the old group might be associated with slips in a more lateral direction. SignificanceOur findings suggest that older adults have a high risk of slipping in a more lateral direction. Shoes with high-slip resistance in the lateral direction are recommended to prevent hazardous lateral slips among older adults.

Factorial Analysis of Vertical Ground Reaction Force and Required Coefficient of Friction for Safety of Stair Ascent and Descent

Slipping and falling on ascent and descent stairs often lead to high risk of injuries and fatalities. In this paper, factorial analysis was used to study various factors influential to the biomechanics, such as ground reaction force (GRF) and required coefficient of friction (RCOF). The purpose of this study is to investigate the effect of the factors, such as stairs, climbing style, climbing mode and participants on vertical GRF and RCOFs.

Characterizing the Required Friction during Ladder Climbing

This study developed a method to quantify required coefficient of friction (RCOF) during ladder climbing. This method was then used to assess the impact of ladder angle on slip risk. Just like in gait, we assert that slip risk during ladder climbing could be determined using RCOF. Subjects climbed a ladder positioned at three angles (90°, 82.75°, and 75.5°). Forces were measured using a force plate and then projected onto the orientation of the shoe-rung contact region to determine shear and normal forces. The maximum RCOF was determined from the force plate forces and shoe orientation with a set of criteria. The maximum RCOF for a ladder set at 90° and 82.75° was significantly larger than from the maximum RCOF for a ladder set at 75.5°. The maximum RCOF increased as the ladder became steeper, indicating climbers may be more susceptible to slipping while climbing a steeper ladder.

P 081 - Required friction curve patterns in a child with unilateral spastic cerebral palsy and true equinus gait

The aim of this study was to describe the required coefficient of friction (RCOF) curve pattern of a child with unilateral spastic cerebral palsy (USPC). A child with USCP and true equinus and a healthy age-matched child participated in this study. They walked barefoot at a self-selected pace along a carpeted path and over two force platforms. RCOF was obtained as the ratio between the tangential forces (FT), and the vertical ground reaction force, (FZ). The RCOF curve present different patters when the USCP child was compared with the control, mainly for the affected side where it was observed lower RCOF peaks and valley as respected the nonaffected side and control values.

Decrease in required coefficient of friction due to smaller lean angle during turning in older adults

We investigated age-related differences in the required coefficient of friction (RCOF) during 90° turning, the difference of RCOF during step and spin turn, and how affects observed differences. Sixteen healthy young and healthy older adults (eight men and eight women in each group) participated. Participants performed 90° step and spin turns to the right at a self-selected normal speed. Older adults turned with lower RCOF than the young adults during both step and spin turns. This was associated with reduced mediolateral (ML) RCOF component (RCOFML) for the older adults. Reduced RCOFML in older adults was associated with reductions in the ML component of the lean angle of the body during turning. This age-related gait changes during turning can be compensatory mechanisms that allowed older adults to turn while reducing the risk of slipping. Spin turns exhibited lower RCOF, resulting from significantly lower RCOFML, than step turns in young and older adults; thus, spin turning is a safer turning strategy for preventing lateral slips. This may suggest that, in older adults, slip prevention may take precedence over balance recovery after slips sustained during turning. These results illustrate a turning gait mechanism that helps prevent slips and falls, and how age affects this mechanism.

Coefficient of friction testing parameters influence the prediction of human slips

Measuring the available coefficient of friction (ACOF) of a shoe-floor interface is influenced by the choice of normal force, shoe-floor angle and sliding speed. The purpose of this study was to quantify the quality of slip prediction models based on ACOF values measured across different testing conditions. A dynamic ACOF measurement device that tests entire footwear specimens (Portable Slip Simulator) was used. The ACOF was measured for nine different footwear-contaminant combinations with two levels of normal force, sliding speed and shoe-floor angle. These footwear-contaminant combinations were also used in human gait studies to quantify the required coefficient of friction (RCOF) and slip outcomes. The results showed that test conditions significantly influenced ACOF. The condition that best predicted slip risk during the gait studies was 250 N normal force, 17° shoe-floor angle, 0.5 m/s sliding speed. These findings can inform footwear slip-resistance measurement methods to improve design and prevent slips.