Table 3: British Standard Roller Chain Selection Table
STEP 5 – Obtain the number of pitches of chain (L).
Table 2 – Multi-Strand Factor
Number of Chain Strands
Multi-Strand Factor
Double Strand Triple Strand
1.7 2.5
Number of Pitches of Chain
( N 2 − N 1 2
6.28 )
2
= 73.35 = 74 Links of chain
+2 C + ( N 2 − N 1
6.28 )
N 1 + N 2 2
L = N 1 + N 2 2
Installation and Arrangement
C
+2 C +
L =
C
Any fraction of L is counted as one pitch.
Speed Ratio and Chain Wrap The speed ratio of the roller chain can range up to 7:1 under normal operating conditions. However, a speed ratio of 10:1 is possible if the speed is very low. Chain wrap on a small sprocket must be at least 120 degrees. Distance Between Shafts Sprockets can be separated by any distance as long as their teeth do not touch. Optimum distance is 30 to 50 times the pitch of the chain used except when there is a pulsating load. In such cases, the distance should be up to 20 times the pitch of the chain used. Position Roller chains are ideally installed horizontally. When chains are installed at angles approaching the vertical, they stretch quickly and may slip off the sprockets. In such cases, the sprockets should be adjusted properly. Slack Generally, the slack of a roller chain should be on the lower side (Figure A). Adequate slack is 4% of the span for normal drives. In the following cases, the slack should be about 2% of the span: a) Vertical Drive or close to vertical drive. b) Center distance between two shafts is greater than 3ft. c) Chain is operated under heavy load and high frequency of an on and off drive. d) Direction of the drive is often changed.
Center Distance in Pitches
2 }
" C = 1 8 {
2 L − N 1 − N 2 + ( 2 L − N 1 − N 2 ) 2 − 8 9.86
1 8 {
2 }
2 L − N 1 − N 2 + ( 2 L − N 1 − N 2 ) 2 − 8 9.86 ( N 2 − N 1 )
" C =
( N 2 − N 1 )
Example: Data: 1. Type of application: Centrifugal Blowers 2. Source of Power: Electric Motor 3. HP to be transmitted: 40 hp
L: Number of pitches N 1 : Number of teeth (small Sprocket) N 2 : Number of teeth (large sprocket) C: Center distance in pitches
4. Drive shaft: 600 rpm 5. Driven shaft: 200 rpm 6. Center distance: 19 inches 7. Space Limit: Maximum 24 inches STEP 1 – Service Factor per Table 1 – 1.0 STEP 2 – Calculate HP Design HP = HP transmitted●SF = 40 HP●1.0 = 40 HP STEP 3 – Obtain chain size and the small sprocket teeth count from the selection table for 40 hp and 600 rpm. According to the selection table, the selected chain and sprocket RPM’s are: * For (a), the necessary number of teeth for both small and large sprockets are 25 teeth and 75 teeth respectively, since the speed ratio is 1/3 (200/600 rpm). But the outside diameter of the sprockets * For (c), the necessary number of teeth for small and large sprockets is 25 (O.D. 8.4 inches) and 75 teeth (O.D. 24.4 inches) respectively. It exceeds the space limitation again (8.4 inches + 24.4 inches > 24 inches). * For (b), the necessary number of teeth for the small and large sprockets is 17 (O.D. 5.9 inches) and 51 (O.D. 16.8 inches). It satisfies the space limitation (5.9 inches + 16.8 inches < 24 inches). A combination of RS16B-2, and 17 teeth and 51 teeth must be used to fulfill all the necessary requirements. STEP 4 – Use Table IV to determine the lubrication method (a) RS12B-3 chain and 25 tooth sprocket (b) RS16B-2 chain and 17-tooth sprocket (c) RS16B chain and 25-tooth sprocket
Chain Speed
Small sprocket revolution speed r/min
P x N x n 12
S=
= (ft./min)
S: Chain Speed (ft./min.) P: Chain Pitch (Inch) N: Number of sprocket teeth n: RPM of sprocket
2 The selection table is based on the following conditions: 1. The chains are operated under ordinary conditions. The ambient temperature range is between 15°F and 140°F. They are not to be in an environment where abrasive dust or corrosive gas is present or when the humidity is high. 2. The two transmission shafts are in a horizontal position and the chains are properly installed. 3. The load does not change significantly during transmission. The “Service Factors” given in Table 1 are used under various operating conditions. The load conditions will affect the chain life. The increase in horsepower rating of the multi-strand chains cannot be calculated by multiplying the horsepower rating of one strand by the total number of strands since the load on each strand is not the same. In order to estimate the service life of a multi-strand chain, the “Multiple-Strand Factor” given in Table 2 must be used. 4. Refer to chain lubrication styles and details in Technical Data sections.
Chain Tension from HP
30,000 x HP S
T=
= (lbs.)
T: Chain Tension (lbs.)
Position of Sprocket The two shafts should be parallel and preferably in a horizontal position. Sprockets should be firmly installed (Figs B & C). Use a straight edge to check that the two sprockets are installed along the same horizontal plane.
P x N x n 12
Chain Speed (S):
1 x 600 x 17 12
=
= 850 ft/min
System B is suggested
255
256
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