Kathryn L. Franklin

Accurate assessment of the brake torque on a rope-braked cycle ergometer

A Monark cycle ergometer is a device globally used in physiological studies to measure the work and energy levels of exercising humans. In this paper a rope-braked cycle ergometer is examined to determine accurate measures of work and power. The work done is generally assumed, by physiologists, to be the load (masses suspended in a basket to apply tension to the rope) multiplied by the distance through which a flywheel braked by the load is moved (Åstrand). In this case the flywheel of the ergometer is designed such that any point on the circumference moves through 6 m for one complete revolution of the pedals. This is a simplistic approach and fails to take into account rope-brake theory and mechanical analysis of the brake mechanism. The dynamic coefficient of friction between the steel flywheel and nylon rope was determined experimentally to be 0.175. The physical dimensions of the ergometer brake system were taken and the theoretical brake torque calculated for a series of loads. It was found that this calculated brake torque was 10.8% less than the assumed brake torque. This error means that the work and power measurements obtained from the ergometer are overestimated by 10.8% for steady speed tests.The braking force was measured experimentally using specially designed load cells to determine the tensions in the brake rope. The experimental results confirmed that the brake force is the same as that predicted using rope-brake theory and that the values of power and work used by sports scientists and physiologists are overestimated by approximately 11%.

Assessing Accuracy of Measurements for a Wingate Test Using the Taguchi Method

The purpose of this study was to establish the effects of four variables on the results obtained for a Wingate Anaerobic Test (WAnT). This study used a 30 second WAnT and compared data collection and analysed in different ways in order to form conclusions as to the relative importance of the variables on the results. Data was collected simultaneously by a commercially available software correction system manufactured by Cranlea Ltd., (Birmingham, England) system and an alternative method of data collection which involves the direct measurement of the flywheel velocity and the brake force. Data was compared using a design of experiments technique, the Taguchi method. Four variables were examined - flywheel speed, braking force, moment of inertia of the flywheel, and time intervals over which the work and power were calculated. The choice of time interval was identified as the most influential variable on the results. While the other factors have an influence on the results, the decreased time interval over which the data is averaged gave 9.8% increase in work done, 40.75% increase in peak power and 13.1% increase in mean power.

Further Mechanical Considerations Between Polar and SRM Mobile Ergometer Systems During Laboratory-Based High-Intensity, Intermittent Cycling Activity

The purpose of this article is to outline mechanical issues related to the use of the Polar S710 heart monitor with Power Unit when compared with the SRM Powercrank system. There are issues outlined in this article that refer to the suitability of the Polar S710 for the quantification of performance during downhill cycling that relate to chain vibration, chain tension, and time interval sampling rates.

Comparison of methods for determining power generated on a rope-braked cycle ergometer during low-intensity exercise

This study compares the instrumentation and analysis techniques used when determining the power expended pedalling a rope-braked ergometer manufactured by Monark (Sweden) during a low intensity test. Power values were generated by eight subjects. The instrumentation consisted of load cells to measure the rope brake forces, a tachometer to measure the flywheel velocity and instrumented pedal cranks manufactured by Schoberer Rad Messtechnik (SRM). The subjects pedalled a rope-braked ergometer at 60 rev min-1, against a resistance of 3 kg, for 5 minutes. Three different measurements of the mean power were recorded and these were compared with the value given by Monark. The SRM cranks provided two sets of results using different software packages supplied with the cranks. SRM standard software is used for taking measurements during training and cycle races over long time periods. An additional piece of software is provided by SRM called Ptnew, which gives readings of torque and pedal cadence over periods up to 30 seconds. Using the values supplied by Monark each subject generated 180 W of power. The mean power for the eight subjects, measured using the SRM cranks, was 170.36 W (SD 4.11) using the alternative SRM software (Ptnew) over a 30 second period and 173.68 W (SD 2.21) using the standard SRM software. From the direct measurement of the brake forces and flywheel velocity the mean power across the eight subjects was 148.90 W (SD 5.89). The SRM cranks measure the input power, whereas the direct measurement system measures the power output excluding mechanical losses. These values give a figure for the mechanical efficiency for the roped-braked ergometer of 88%. It was found that Monark overestimates the power generated by the subjects when compared with both the SRM systems and the direct measurement instrumentation.

Finite Element Simulation of Ice Axe Pick Impact on a Semi-Rigid Surface

The aim of this paper is to investigate, using finite element analysis, a possible cause of fatigue failure due to the pick of an ice axe impacting a semi-rigid surface This simulates the pick striking a thin layer of ice covering rock. The technical ice axes used by climbers for ice wall climbing are subjected to a number of different loading situations in use. These can result in high stresses being generated. The geometry of the ice axe pick consists of a series of teeth along the bottom surface, which bite into the ice to give the mountaineer purchase when climbing. The tooth profile is such that, at the root, they would act as points of high stress concentrations in the presence of tensile stresses. However the stresses at the root of the tooth are generally compressive for most load cases. However, there have been recorded instances where an ice axe pick has failed in use due to fatigue with the crack being initiated at the root of a tooth. The requirement for a fatigue failure is a cyclic load which has at some point a tensile stress present. As part of an initial study a transient dynamic finite element analysis was carried out on the geometry of an ice axe pick striking a semi-rigid surface. The pick was assumed to strike at velocity of 9m/s at an angle to the tip of 20°. It was assumed that the pick was manufactured from steel and both elastic and plastic properties modelled. The impact surface was given the physical properties of granite and was considered to be flat. A two-dimensional analysis was carried out. Only the pick of the axe was modelled, the adze and handle ignored. It was assumed that the pick was rigidly held at the junction with the ice axe handle. To model the impact the surface was given an initial velocity and a mass of 0.7kg, which was the total mass of the ice axe. The analysis revealed that a compressive failure occurred at the tip of the ice axe pick causing the characteristic rounding of the pick tip (blunting). The stress at the root of the teeth nearest to the tip of the pick yielded in compression, which resulted in a tensile residual stress in the unloaded pick. Each successive impact would therefore result in a tensile-compressive stress cycle which is conducive to fatigue failure.

Finite Element Simulation of Ice Pick Torquing (P97)

The ice axe is an essential tool for any mountaineer climbing in icy conditions. Ice axes are often used for climbing frozen waterfalls and icy rock faces. One of the techniques used in such climbs is known as torquing. This technique involves placing the pick of the ice axe into a crack and pulling on the ice axe handle to wedge the pick in the crack to give purchase for the climber. This technique was initially frowned upon by ice axe manufacturers as the pick was not designed for such loading, but it has become an accepted climbing technique and the manufacturers have modified their product accordingly. There have been a number of ice pick failures due to fatigue, where the crack was initiated at the root of the ice pick tooth. It has been suggested that torquing is responsible for these failures. In this study the ice axe pick was modelled using three dimensional elements. Two cases were analysed: the first a static finite element analysis assuming the tip of the pick is clamped and the second a transient analysis modelling the tip in a crack in the rock. The results showed that in the static case the highest stresses were not at the root of a tooth, but there were tensile stress concentrations in the root of the teeth nearest the handle. The transient analysis revealed a high shear stress at the root of the two teeth nearest the axe handle. In both cases these may result in the fatigue failure of an ice pick with the crack initiating at the root of a tooth.

Mechanics of a Rope-Braked Cycle Ergometer Flywheel and its use for Physiological Measurement

The cycle ergometer is one of the main tools used by physiologists in studies involving the measurement of work or power against physiological criteria. The current mechanical analysis of a Monark 824E rope-braked ergometer is based on a simplified approach. In this study a detailed analysis of the system is adopted to increase understanding. The mechanics of the ergometer flywheel are explained using data generated experimentally and the detailed components of the values of work that comprise the total work done by a subject are discussed. The total work done by a subject in 2.5 minutes at a speed of 60 rpm against a brake mass of 3 kg was 23,034 J compared with the value of 27,000 J that normally would be attributed using the traditional calculations. This 15% difference is mainly due to the incorrect assumption that the brake load is the force due to the basket mass.