Belt Friction 2 PDF

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Summary

Uses kinematic and dynamic analysis to find frictional coefficient for belt....

Description

Experiment: Determine belt friction using belt friction apparatus.

Objective: The object of the experiments is to test flat belt with varying angles of lap around a pulley. Then the empirical data may be compared with the theoretically derived solutions and the coefficients of friction evaluated for all the belts types.

Apparatus: Apparatus (LAB-MECH-ET-13) consists of a pulley mounted upon ball bearings. The pulley can accept flat, vee or round belts. The belt under test is placed over the spring balance hook. The spring balance is hooked to the extensi on arm which is secured to the back-plate to give the desired angle of lap. The main load is applied to the bottom end of the belt via a load hanger. The torque required to just make the belt slip may be determined by looping the cord over the small peg in the pulley periphery and adding weights to the load hanger until slip just occurs.

Theory: Flat belts have been used for power transmission for many centuries. They are simple and reliable with the capacity to operate for long periods without mainten ance. Belt drives have two distinct differences from a gear drive. The first is that adjacent shafts can rotate in the same direction without the use of an intermediate or idler, as is necessary on a gear train. The second is that some slip is possible if the load suddenly increases. A belt can be used to transmit power with both sides enabling very efficient and compact drives in e.g. packaging machinery or textile plant.

The maximum power which driver pulley could transfer can be transmitted when the belt is on the point of slipping. So at the moment of slippage belt is at highest point of friction. At momentary analysis velocity is constant for that moment. Thus we require knowing the relationship between the tensions T1 and T2 with respect to the pulley. The difference in these tensions is the force applied to the pulley at its periph ery hence the torque and power transmitted. Resolving vertically we obtain:- T + dF so:-

( T + dT ) = 0

dF = dT

(1)

That is the increment of friction developed over the length r dß and is equal to the increment of tension in the belt over the same length. Resolving horizontally we obtain; remembering that as dß is small sin dß = dß. dN

T

( T + dT )

=0

(2)

Neglecting small quantities of second order yields: dN = T dß

(3)

This equation gives us the element of normal pressure at any point on the belt in terms of the tension T in the belt at that point. At the point of slipping: dF =

dN

(4)

Substituting for these quantities from the expressions above, we find:dT =

Tdß

dT / T =

dß

(5)

If we now integrate the above expression over the entire belt contact arc we can find the ratio of the belt tensions.

= ln T1/T2 =

(6)

T1/T2 = e This gives the ratio between the tensions on either side of the pulley. It shows that it increases very rapidly with the angle of lap, .

Procedure: Set up the flat belt on the apparatus and place a load on the hanger. Place the cord on the pulley peg in a clockwise direction and add its load hanger. Gently add weights to the cord hanger until the pulley is on the point of slipping. Note the two hanger loads i.e. hanger attached with belt (T1) and Load Hanger (Tcord) and the spring balance reading (T2). To check the spring balance reading remove the load applied via the cord and try to rotate the pulley by hand until the point of slip is reached. This may not be possible at higher loads so knowing the load required for slip gradually re-apply it to the cord but carefully observe the spring balance. Repeat the experiment for each angle of lap from 30 to 90 degree.

Safety Precautions: No eating or drinking in the lab. Be careful while handling heavy equipment. Do not perform unauthorized experiment by yourself. Never leave experiment that is in progress unattended. No Laboratory Work should be carried out in the absence of instructor. Shoes should not be slippery in order to avoid any slippage of student on lab floor....