Machine Design

Metal Chain Drives

Metal-chain drives are normally used for applications below 3,000 rpm where accuracy and reliability must be greater than that provided by rubber belts. Because a metal chain does not stretch or slip as a belt does, chain drives maintain constant speed ratios under widely varying load conditions and need adjustment infrequently.

Reliability of a chain drive is superior to that of belts, with chain service life typically rated at 10,000 hr, about three to four times longer than that of belts. Also, chain drives generally are more compact than similarly rated belt drives. For example, a 10-hp chain drive is about 50% smaller than a comparable belt drive.

Chains can be replaced more easily than belts because sheave adjustment is not changed for the replacement. Chains can be removed by simply pulling out a master pin in the link and threading the new chain through the drives. But because chain drives do not slip, they cannot provide the overload and jam protection of belt drives. Also, the heavy weight of metal chain compared with light rubber belts produces penalties in efficiency, cost, and response time. Efficiency for chain drives -- about 85 to 90% -- is slightly less than that of conventional belt drives. And chain drives cost about 20 to 50% more than belt drives, with prices ranging from about $300 to $10,000 for 0.70 to 100-hp models.

Chain drives use two types of links: self-forming teeth andextended pin. Self-forming teeth have slats that extend on each side of the link and conform to the ribbed sides of the sheave faces for a positive engagement. This type usually transmits up to 25 hp and operates at input speeds to almost 900 rpm. The extended-pin type, with pins projecting on both sides of the link to engage the sheave face, transmits from 7.5 to 100 hp at input speeds to 2,900 rpm. In the range of 7.5 to 25 hp, ratings for the two link types overlap, with prices comparable in the 7.5 to 15-hp range and with extended-pin units being less expensive in the 20 to 25-hp bracket.

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