1 1. General
The chains are made up of number of rigid links which are hinged together by pin joints in order to provide the necessary flexibility for wrapping round the driving and driven wheels. These wheels have projecting teeth of special profile and fit into the corresponding recesses in the links of the chain as shown in Figure 1. The toothed wheels are known as sprocket wheels or simply sprockets. The sprockets and the chain are thus constrained to move together without slipping and ensures perfect velocity ratio.
These chains are used for transmission of power, when the distance between the centers of shafts is short.
These chains have provision for efficient lubrication.
2. Types of Power Transmission Chains 2.1 Block or bush chain.
This type of chain was used in the early stages of development in the power transmission.
2.2 Bush roller chain.
This consists of outer plates or pin link plates, inner plates or roller link plates, pins, bushes and rollers. A pin passes through the bush which is secured in the holes of the roller between the two sides of the chain. The rollers are free to rotate on the bush which protects the sprocket wheel teeth against wear. The pins, bushes and rollers are made of alloy steel.
J. CHAINS - LECTURE
2
2.3 Silent chain.
A silent chain (also known as inverted tooth chain) is designed to eliminate the evil effects caused by stretching and to produce noiseless running. When the chain stretches and the pitch of the chain increases, the links ride on the teeth of the sprocket wheel at a slightly increased radius. This automatically corrects the small change in the pitch..
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3 3. Terms and Definition
3.1 Pitch of chain (P)
It is the distance between the hinge centre of a link and the corresponding hinge centre of the adjacent link.
3.2 Pitch circle diameter of chain sprocket (D).
It is the diameter of the circle on which the hinge centers of the chain lie, when the chain is wrapped round a sprocket.
4. Designation of Roller Chains Chain
no. 25 35 41 40 50 60 80 100 120 140 160 180 200 240
Pitch ¼ 3/8 ½ ½ 5/8 ¾ 1 1 ¼ 1 ½ 1 ¾ 2 2 ¼ 2 ½ 3
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4 5. Tabulated Horsepower Ratings for Roller Chain Drives
Refer to PSME Code 2008,, Table 3.11, page 27 and other references.
6. Types of Lubrication.
Type I – manual lubrication – oil is applied periodically with a brush or spout can, preferably at least once every 8 hours of operation, (vmax = 300 fpm).
Type II – drip lubrication – oil drops are directed between the link plate edges from a drip lubricator, (vmax = 1300 fpm).
Type III – bath or disc lubrication – with bath lubrication the lower strand of chain runs through a sump of oil in the drive housing. The oil level should reach the pitch line of the chain at its lowest point while operating. With disc lubrication, the chain operates above the oil level. The disc picks up oil from the sump and deposits it onto the chain, usually by means of trough. (vmax = 2300 fpm).
Type IV – oil stream lubrication – the lubricant is usually supplied by a circulating pump capable of supplying each chain drive with a continuous stream of oil.
7. Pitch and Pitch Circle Diameter
=
Nt
D
P 180
sin Where:
D = Diameter of the pitch circle, P = Pitch, and
Nt = Number of teeth on the sprocket.
8. Velocity and Velocity Ratio of Chain Drives The velocity ratio of a chain drive is given by
1 2 2 1
N N n mw=n =
where
n1 = Speed of rotation of smaller sprocket in r.p.m., n2 = Speed of rotation of larger sprocket in r.p.m., N1 = Number of teeth on the smaller sprocket, and N2 = Number of teeth on the larger sprocket.
The average velocity of the chain is given by nfpm
PN
vm Dn t
12 12 ≈
=π where
D = Pitch circle diameter of the sprocket in inches, and P = Pitch of the chain in inches.
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5 PNt = circumference of the sprocket.
9. Length of Chain and Centre Distance
The center distance for chain drives may of course be relatively short, but a minimum wrap of 120o is desirable;
this condition is inevitably met when mw < 3. An average good center distance would be D2 + D1/2, where D2 is the pitch diameter of the larger sprocket, D1 of the smaller. The approximate length of chain is
( )
, pitches C
N N N C N
L 2 40
2
2 1 2 1
2 −
+ + +
≈
Where C is in pitches. The length should be an even number of pitches to avoid using an offset link. This is the usual matter of adjusting chain length, center distance, and sprocket sizes so that everything fits.
10. Center Distance for Given Length in mm or inches.
( ) ( )
− − + − − − −
= 2 1 2
2 1 2 1
2 2 0810
2
8 L N N L N N N N
C P .
11. Number of Teeth on the Smaller or Driving Sprocket or Pinion
For very low speeds, the recommended minimum number of teeth on the smaller sprocket is Nmin = 12;
for low speeds, Nmin = 17; for moderate speeds, Nmin = 21; for high speeds, Nmin = 25;
for speed increasing drives, Nmin = 23.
With odd tooth numbers on the smaller sprocket and an even number of pitches in the chain, the frequency of contact between a particular tooth and a particular roller is a minimum, presumable better distributing the wear.
12. Maximum Pitch
3 2
900
≤ p n
Where p is the pitch in inches and n is the speed of the small sprocket.
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6 13. Recommended Lubricant Viscosities.
Temperature, F Viscosity 20 to 40 SAE 20 40 to 100 SAE 30 100 to 120 SAE 40 120 to 140 SAE 50 14. Determining the Number of Strands
14.1 For a given transmitted power, get the design power by multiplying it by the service factor.
14.2 Find the horsepower rating per strand from tables.
14.3 Divide the design horsepower by the horsepower rating per strand to get the number of strands. Use a certain factor as a function of the number of strands if any from tables.
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