4.1.4 Power Transmission Chain Selection for Slow Speeds
This selection procedure is based on the maximum allowable tension, which is used when the chain speed is less than 50 m/min., and the starting frequency is less than 5 times/day. The selection is done following the flow chart in Figure 4.11.
EXAMPLE: Recalculate the previous example from Basics Section 4.1.3 based upon the selection for slow speed.
- Step 1. Tentatively select RS120 chain, which is one size smaller than RS140, and a 15-tooth sprocket. Then calculate the chain speed.
V = PNn / 1,000 = (38.1 × 15 × 50)/ 1,000 = 28.6 m/min. < 50
According to this speed and starting frequency, case selection for slow speed may be used.
- Step 2. From the rated power of the motor, calculate the tension Fm on the chain.
Fm = 60 × kW / V = 60 × 7.5 / 28.6 = 15.7 kN
- Step 3. Service factor Ks = 1.3, Chain speed coefficient Kv = 1.06
(from the chain speed 28.6 m/min.).
- Step 4. Sprocket tooth coefficient Kc = 1.27 (from 15-tooth sprocket).
- Step 5. Calculate the design chain tension F′m.
F′m = Fm × 1.3 × 1.06 × 1.27 = 27.5 kN
- Step 6. Decide on the chain size.
According to the catalog, the maximum allowable load of RS120 is 30.4 kN. Because this value is higher than the chain design tension determined in Step 5, RS120 may be used in this application.
Select the number of teeth in the large sprocket according to the speed ratio, using the same procedure as in the general selection.
Confirm the chain and the sprocket: driving sprocket is RS120-15T (maximum hub bore is 80 mm, and the shaft diameter is 66 mm; therefore, this may be used), and driven sprocket is RS120-38T (maximum hub bore is not shown in catalogs). Therefore, consult with the manufacturer and determine that the 38-tooth sprocket will accommodate a 94-mm shaft.
- Step 7. Calculate the chain length (number of links).
C = 1,500 ÷ 38.10 = 39.37
L = (N + N′) / 2 + 2C + ((N − N′) / 6.28)2 / C = (38 + 15) / 2 + 2 × 39.37 + ((38 − 15) / 6.28)2 / 39.37
= 105.58 links
Therefore, use 106 links. Center distance = 1,508 mm.
- Step 8. Check the manufacturer's catalog to determine the necessary type of lubrication (manual or drip).
As you see, this selection allows you to choose a smaller and more economical chain than the general selection. But, at the same time, consider these facts:
- Do not use offset links or normal connecting links for slow speeds. Use tap fit connecting links, which have a tight interference fit. If you want to use offset links or normal connecting links, check the strength derating shown in Basics Section 2.2.3 and recalculate.
- Cast-iron sprockets are not strong enough for slow speeds. Therefore, use SS400, S35C, S45C, etc.
- Use a hardened-tooth sprocket for the high-speed sprocket.
- The bearing pressure on the chain will be very high, so lubricate the chain well.