INSTRUCTION MANUAL HFN.15. Slipping Friction Apparatus

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1 INSTRUCTION MANUAL HFN.15 Slipping Friction Apparatus

INTRODUCTION The study of friction has to take account of two different circumstances, namely the static case and the sliding or slipping

The measurement of coefficients of friction is of great importance in the design of moving machinery because friction is a source of heat and a

This has led to the production of several ways of measuring friction in both artificial and practical pieces of apparatus, some of which are

In most cases the measurement of friction is made over a few seconds while the two surfaces rub against each other, and it might be thought that

2 INTRODUCTION The study of friction has to take account of two different circumstances, namely the static case and the sliding or slipping situation. The measurement of coefficients of friction is of great importance in the design of moving machinery because friction is a source of heat and a cause of wear. This has led to the production of several ways of measuring friction in both artificial and practical pieces of apparatus, some of which are available in the Hi-Tech range of tribology experiments. In most cases the measurement of friction is made over a few seconds while the two surfaces rub against each other, and it might be thought that this lacked in accuracy because a steady state was not achieved. The apparatus used for this experiment is an ingeneous attempt to counter the above criticism by continuously subjecting the two surfaces to a condition of slipping friction. The design involves a clever application of simple harmonic motion as a rod supported on two contra-rotating wheels oscillates longitudinally. The success, or otherwise, of the invention can be gauged by canying out the experiment. HFN.15. Page I. rsue I. July

LIST OF PARTS The standard set of items supplied (HFN 5) consists of: - Slipping friction apparatus - Mild steel rod, 16 mm diameter x 0.6 m long - Mild steel tube, 16 mm old x 0.

wrench A stop watch is required to perform this experiment APPARATUS The principal feature of the apparatus is a pair of V grooved mild steel wheels mounted in the same vertical plane and driven at

The other wheel shaft passes through a threaded bush which is clamped in a long horizontal slot by a capstan nut.

3 LIST OF PARTS The standard set of items supplied (HFN 5) consists of: - Slipping friction apparatus - Mild steel rod, 16 mm diameter x 0.6 m long - Mild steel tube, 16 mm old x 0.6 m long - Mild steel tube, 32 mm old x 29 mm i/d x 0.6 m long - Brass rod, 16 m diameter x 0.6 m long - Aluminium alloy rod, 25.5 mm diameter x 0.6 m long - 3 mm hex. wrench A stop watch is required to perform this experiment APPARATUS The principal feature of the apparatus is a pair of V grooved mild steel wheels mounted in the same vertical plane and driven at the same speeds in opposite directions. One wheel is in a fixed bush in a vertical steel plate. The other wheel shaft passes through a threaded bush which is clamped in a long horizontal slot by a capstan nut. A variable speed electric motor with a worm gear reduction drives the movable wheel by a toothed belt and the fixed wheel through a 1: 1 spur gear to reverse its direction of rotation. An adjustable jockey wheel takes up the slack as the movable wheel is re-positioned, but to cope with the large range of centres distance a long and a short toothed belt are provided. A removable belt guard covers the drive mechanism. A set of test specimens is supplied to help verify the theory. Ideally for comparison of results the three mild steel specimens ought to have identical surfaces, but that is difficult. The main variables are the coefficient of friction, the size and the weight of the specimens. The whole apparatus is built on a heavy steel base with levelling feet. At the left hand end is the speed controller housed in a box which includes an illuminated push switch for the power supply. EXPERIMENT It is important to degrease and clean the test specimens and V grooves in the wheels before and during the experiment. Between being used the wheels and specimen should be protected against corrosion. [.\'SUe HFN./5. Page 2., I. August. /993.

OBJECT The object of this experiment is to check the suitability of the apparatus to verify the expression for periodic time of the oscillatory m6tion This entails I.

showing the effect of r, the height of the centre of gravity of the specimen above the contact with the wheels 4. determining the coefficient of friction J.l of different materials on mild steel.

The jockey wheel set should always be rotated clockwise (as viewed from the front of the apparatus) when taking up slack in the belt.

Find the mass of each test specimen and record it in table 1 Table 1 Code A B - C L e Material MildSteelnxl Mild Steel tube Mild Steel tube B rass nxi Aluminium Allov rod Dimensions 16 mm diameter x

6 m long Mass The simplest method of performing this experiment is to set the wheels for each distance t and use all the specimens in turn before changing to the next value of t.

4 OBJECT The object of this experiment is to check the suitability of the apparatus to verify the expression for periodic time of the oscillatory m6tion This entails I. showing that t is independent of the weight of the specimen 2. showing that f is proportional (nearly) to the centres distance / between the wheels 3. showing the effect of r, the height of the centre of gravity of the specimen above the contact with the wheels 4. determining the coefficient of friction J.l of different materials on mild steel. PROCEDURE It will be found convenient to use the centres distances I of 0.2 and 0.25 m with the shorter toothed belt and 0.3 and 0.35 m with the longer belt. The jockey wheel set should always be rotated clockwise (as viewed from the front of the apparatus) when taking up slack in the belt. To change the belt remove the belt guard at the back of the apparatus. Find the mass of each test specimen and record it in table 1 Table 1 Code A B - C L e Material MildSteelnxl Mild Steel tube Mild Steel tube B rass nxi Aluminium Allov rod Dimensions 16 mm diameter x 0.6 m long mm old x 0.6 m long 32 mid old x 0.6 m long mm diameter x 0.6 m long 25.5 mm diameter x 0.6 m long Mass The simplest method of performing this experiment is to set the wheels for each distance t and use all the specimens in turn before changing to the next value of t. The results can be separated afterwards to study the four objects. Set the highest value of 1=0.35 m and adjust the jockey to make the belt firm. Turn the speed control down to the lowest number and switch on the power. Turn the speed control up until the wheels rotate steadily and then lay specimen A on the wheel grooves. Vary the speed to make the specimen oscillate steadily to left and right. It may be necessary to give the rod a small push to start it oscillating. Try to achieve a steady oscillation of around ::t5 cm amplitude, and use a stop watch to time 10 cycles. Repeathis thrice more and record this in table 2. Change the specimen and repeat the procedure. The required speed will change for different specimens. If a damped oscillation is all that can be attained time as many cycles as possible and record all the data. HFN.15. Page 3. Issue I. July

5 Table 2 Clean each specimen after use, and clean the grooved wheels frequently. RESULTS The theory on which this experiment is based is as follows. Consider a unifonn rod supported on two wheels rotating at the same speed in opposite directions as shown. I 2.. x '" { ~ f..~cr ~,J; c~.~ tiji w ~ fa When the centre of gravity of the rod is displaced x to the right of the mid-point between the wheels, let the reactions due to the weight of the rod be RA and RB- Then if J.1 is the coefficient of sliding friction there is a force returning the rod to a symmetrical position of J.1(RB - RA). Taking moments about the centre of gravity for instantaneous equilibrium of the rod which yields R R - Wx cc" B- A-I. ~ HFN. 15. Page 4. Issue I. August

Hence the restoring force f' is J.1(RB - RA) = ~Wx I'j-=-;:; 1..r- Using Newton's equation f' = ma. JlWX. I ~ 2/- J.

1K- 47t2r g Hence a graph of f against I will give a straight line with a gradient m= 27[2 Jlg and an intercept of (g= 9.81 mls2 In this experimenthe value of r is 0.016 -+ 0.

It may be seen at this stage that there are considerable irregularities in these values which will make interpretation difficult.

The results for the mild steel specimens A, B and C should be plotted on graph I on the assumption that the coefficient of friction is the same for the three specimens.

proportional to / - object (2). It would be useful to plot the /=0 intercept, given above, to provide one definite point for the straight line(s) through the results.

6 Hence the restoring force f' is J.1(RB - RA) = ~Wx I'j-=-;:; 1..r- Using Newton's equation f' = ma. JlWX. I ~ 2/- J.1r W d2x -- g dt2 The solution of this differential equation gives from which f = ~ J.1K- 47t2r g Hence a graph of f against I will give a straight line with a gradient m= 27[2 Jlg and an intercept of (g= 9.81 mls2 In this experimenthe value of r is which will be found to be roughly negligible. The collected results in table 2 should be converted to values of f and entered in table 3. It may be seen at this stage that there are considerable irregularities in these values which will make interpretation difficult. One possibility is to average each set of four values for the chosen values of f on the graphs. The results for the mild steel specimens A, B and C should be plotted on graph I on the assumption that the coefficient of friction is the same for the three specimens. Hence the results of A and B test object (1) - t independent of weight; B and C ought to show the difference due to r - object (3); while all the results should show that f is proportional to / - object (2). It would be useful to plot the /=0 intercept, given above, to provide one definite point for the straight line(s) through the results. Calculate the gradient of the best straight line, and hence derive the coefficient of friction I.l for steel on steel. Use graph II to plot the results for specimens D and E and from the gradients of the best straight lines derive the values of ~ for brass and aluminium on steel. OBSERVATIONS Comment on the values of ~ derived from the experiment. these values? What was the likelihood of achieving object (3)? How could the design of the experiment be improved? Did the results afford confidence in HfN. /5. Page 5. rue I. July. /993.

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