Mechanics of Machines 2 Lab Report (2012)
EXPERIMENT 1: PLATE CLUTCH |
1. INTRODUCTION
Friction is to be avoided in some parts of machines, but is wanted in such items as clutches, brakes and belt-driven pulley systems. The apparatus used for this experiment is a simulation of a clutch, the pressure being provided by vertical weight on a plate. In engaging a clutch between a running engine and a stationary machine there is bound to be some wear, and it is evident that the wear must be uniform on the surface of the friction plate. As wear is proportional to both load and travel it follows that for uniform wear the product of plate pressure and radius from the axis is a constant which depends on the materials.
The substitution of the consequence of assumed uniform wear leads to a simple theoretical expression for the torque transmitted by a plate clutch just before slip occurs. It is evident that this must depend on the friction between the plate and the plate it presses on. Hence this experiment uses three different sizes of friction plates to test the theory and assumption.
2. THEORY
Consider an upper turntable which turns due to the torque T on a concentric friction plate supported by a stationary base.
The inner and outer radii of the plate are ri and ro. Take an elemental plate of radius r and width dr. The axial load induces a localized pressure p which causes a normal force dN, where dN = p.2μr.dr
If the coefficient of friction between the plate and the turntable is μ (assumed to be uniform and independent of r) then the total torsional resistance is
For vertical equilibrium where W is the applied load
As the turntable revolves it seems reasonable to predict that the friction plate thickness will reduce uniformly, that is that the wear rate is constant. Since wear depends on pressure and the rubbing distance then it follows that
p.r = constant (since distance moved is proportional to the radius)
Hence,
3. AIM
The actual purpose of the experiment was to verify, for a single
1. INTRODUCTION
Friction is to be avoided in some parts of machines, but is wanted in such items as clutches, brakes and belt-driven pulley systems. The apparatus used for this experiment is a simulation of a clutch, the pressure being provided by vertical weight on a plate. In engaging a clutch between a running engine and a stationary machine there is bound to be some wear, and it is evident that the wear must be uniform on the surface of the friction plate. As wear is proportional to both load and travel it follows that for uniform wear the product of plate pressure and radius from the axis is a constant which depends on the materials.
The substitution of the consequence of assumed uniform wear leads to a simple theoretical expression for the torque transmitted by a plate clutch just before slip occurs. It is evident that this must depend on the friction between the plate and the plate it presses on. Hence this experiment uses three different sizes of friction plates to test the theory and assumption.
2. THEORY
Consider an upper turntable which turns due to the torque T on a concentric friction plate supported by a stationary base.
The inner and outer radii of the plate are ri and ro. Take an elemental plate of radius r and width dr. The axial load induces a localized pressure p which causes a normal force dN, where dN = p.2μr.dr
If the coefficient of friction between the plate and the turntable is μ (assumed to be uniform and independent of r) then the total torsional resistance is
For vertical equilibrium where W is the applied load
As the turntable revolves it seems reasonable to predict that the friction plate thickness will reduce uniformly, that is that the wear rate is constant. Since wear depends on pressure and the rubbing distance then it follows that
p.r = constant (since distance moved is proportional to the radius)
Hence,
3. AIM
The actual purpose of the experiment was to verify, for a single
References: ii. Fawcett, J.N., 1981, “Chain & Belt Drives – A Review”, Shock Vib. Dig., 13(5), pp.5-12 “On flat belts & Slip”