CHAIN DRIVES Home Chain Drive Notes and Data - Mechanical Design Data Manual & Notes - 3rd Ed; (Chapter 4 - Chains) Latest versions of selection procedures.Renold chain drives

Why use a chain drive?Chain drives have characteristics which, in certain applications, are more favourable than belt drives or gear drives for providing power transmission between two or more shafts. Their major advantage over V-belt drives is that there is no-slip. A V-belt can slip over the pulleys and hence a constant drive speed is not assured. The positive action of the tooth and sprocket in a chain drive means that chain drive assemblies will be more compact than belt drive assemblies.

Chain drives are often preferred to geared drives because they have less rigorous design requirements. For example shaft alignment is less critical for a chain drive. Often the centre distance between shafts is too long for geared drives so a chain drive is used.

Types of Power Transmission Chain:The most common form of chain drive is the roller chain drive found in applications such as the push-bike, motor bike and car timing chain. Another common form is the inverted tooth or silent chain. For specific information on transmission chains refer to the MECHANICAL DESIGN DATA MANUAL.

Design requirements.The most favourable position for a chain drive is with the sprocket centre lines in the same horizontal plane or inclined at an angle up to 60 degrees. Vertical drives should be avoided. Ideally the centre distance between sprockets should be between 30 and 50 times the chain pitch.

30xPitch(P) * Distance between shaft centres (C) * 50xP

Chain drives are designed with centre to centre adjustment.

Wherever possible sprockets should have more than 17 teeth. This is because the chain forms a polygon (not a circle) around the sprocket. If low teeth numbers are used a cyclic drive speed variation will occur which is called cordal speed variation. If this speed variation is not critical to design specification of the equipment then lower teeth numbers may be used at low drive speeds (say less than 3 m/sec). At higher speeds cordal speed variation will cause excessive sprocket wear. Sprockets with 17 to 25 teeth running at speeds greater than 3 m/sec should be heat treated to give a tough, wear-resistant surface with a Rockwell 'C' hardness 35 to 45.DESIGN PROCEDURE

This design procedure complies with BS 228: 1970.

Chain drive design is done by following this procedure and referring to the DESIGN DATA MANUAL page 77.

1. Calculate the drive ratio R (velocity ratio) given the input RPM and output RPM.

2. Select sprocket tooth numbers. In order to reduce costs standard sprockets are used - (Chart 1 p77).

3. Determine the Service (selection) factor. The service factor takes into consideration the conditions under which the chain drive will be working. Shock loads created by the driving machinery and driven load require more robust design. Using Chart 2 in the DESIGN DATA MANUAL determine the type (class) of the driven machinery, then choose the correct column for the driving machinery and determine the selection factor for the number of teeth on the smaller sprocket.

4. Calculate the Design (selection) power.

5. Select chain size from the power rating chart. The design power rating for simplex, duplex and triplex chains are shown in the three columns on the left hand side of the chart. Wherever possible simplex chains are used. The design power on the vertical axis is referenced with the speed of the smaller sprocket on the horizontal axis to obtain a chain size and lubrication requirements.

6. Check the maximum sprocket bores against the required shaft diameters if known. List or tabulate details of stock numbers for chain, sprockets and bushes.

7. Determine a suitable centre distance if not given. As mentioned a centre distance 30 to 50 times the chain pitch is recommended.

8. Determine the length of chain in number of pitches.Round off answer to an even number of pitches.

As the chain must be made up of an even number of pitches, the actual centre distance must be redetermined so that it corresponds to the chain length calculated above.

9. Calculate the actual centre distance CA.

10. Determine the sprocket pitch diameters and other dimensions as required.

PCDsprocket = number of teeth in sprocket * PCD factor

or

WORKED EXAMPLESelect a suitable chain drive to transmit 2.5kW from a geared electric motor running at 200RPM to a rotary kiln running at 80RPM. Assume moderate (medium impulsive) shock loads. Using a centre distance which is twice the pitch diameter of the wheel sprocket, determine:

(a) the length of the chain in pitches,(b) the actual centre distance.

Data:Power transmitted (P) = 2.5kWPrime mover - geared electric motor; RPM=200Driven machine - rotary kiln (medium impulsive load); RPM=80'Anticipated' centre distance (C) = 2*PCDwheel sprocket

Solution:1. Calculate the drive ratio.2. Select sprocket tooth numbers.

n=number of teeth in pinionN=number of teeth in wheel (driven)N=R*nn1719202123

N42.547.55052.557.5

The first available combination is:

n=23,N=57

Exact Drive Ratio (R) =57/23= 2.478 which is within 1% of desired ratio

3. Determine the service factor.Using the table on Chart 2; n=23

Medium impulsive load (class 2) Steady electric motor input

Service factor=1.03

4. Calculate the design power.

P=Theoretical power*service factor P=2.5kW*1.03 P=2.575kW

5. Select chain size.Refer chart 19T pinions Chain: simplex;RPM=200;P=2.575kW.

Chain can be:

15.875mm pitch, manual lubrication 19.05mm pitch, drip lubrication

6. List catalogue information. Chain;Choose 5/8" (15.875mm) lubrication: Class 1 (manual) Reynold chain No 110056.

Pinion;n=23 Number with Plain bore: 213015 (14mm stock, 55mm max) Number with taper bore 213015/9 (Bush No TB1610) PCD=116.59mm

Wheel:N=57 Number with Plain bore: 213042 (24mm stock, 50mm max) Number with taper bore: 213043/9 (Bush No TB2012)

7. Determine centre distance.

Given: C = 2*PCDwheel = 2*288.19 = 576mm

8. Length of chain in pitches.

9. Actual centre distance (CA).

Assignment1. Redo the above question assuming the driven machine is "moderate shock" and the driver is "smooth running". (Looks like this is what should have been chosen in the question above). You shouldget a different application factor than above, and this will change therest of the selection process! (BTW: When you have a choice, you usually avoid duplex or triplex chains because they are more expensive.)