Roller Chain Drive Selection per ANSI B29.1

Chain Sizing: Design Horsepower Method

Chain selection starts with the design horsepower:

HPdesign = HPtransmitted × SF

Where SF is the service factor from the ACA/ANSI application tables. Service factors account for the type of driven equipment and the type of prime mover:

• Smooth load, electric motor: SF = 1.0
• Moderate shock, electric motor: SF = 1.3
• Heavy shock, electric motor: SF = 1.5
• Internal combustion engine (without hydraulic coupling): Add 0.2 to the above

With the design HP and the RPM of the small sprocket, enter the ANSI B29.1 horsepower rating table (published by chain manufacturers like Rexnord, Tsubaki, and Renold) to select the chain size. The rating table gives the maximum HP capacity for each chain pitch at each RPM for a standard 17-tooth small sprocket.

Standard single-strand chain pitches: #25 (1/4"), #35 (3/8"), #40 (1/2"), #50 (5/8"), #60 (3/4"), #80 (1"), #100 (1-1/4"), #120 (1-1/2"), #140 (1-3/4"), #160 (2"), #200 (2-1/2").

If a single-strand chain cannot handle the design HP, use multiple strands. A duplex (2-strand) chain carries 1.7 times the single-strand rating (not 2.0 due to unequal load sharing). Triplex carries 2.5 times.

Sprocket Selection and Speed Ratios

The speed ratio between driver and driven sprockets equals the tooth count ratio:

Ratio = Ndriven / Ndriver = RPMdriver / RPMdriven

General rules for sprocket tooth counts:

• Minimum teeth on small sprocket: 17 is the standard baseline, 12 is minimum for most applications, 9 is the absolute minimum for large-pitch chains. Fewer teeth increase chordal action (speed pulsation), chain wear rate, and noise.
• Maximum teeth on large sprocket: 120 teeth is a typical maximum. Very large sprockets are expensive and subject to tooth wear because each tooth carries load less frequently.
• Odd/even tooth rule: Use an odd number of teeth on at least one sprocket with an even number of chain links. This ensures every tooth engages every link over time, distributing wear evenly.
• Maximum ratio: 7:1 for a single reduction. Beyond 7:1, consider double reduction (two chain drives in series).

Center distance between sprockets should be 30–50 times the chain pitch for best performance. Shorter distances increase the wrap angle on the small sprocket (good) but may limit the number of engaging teeth. Longer distances allow excessive chain sag and whip.

Lubrication: The Most Neglected Factor

Chain lubrication is not optional. An unlubricated chain running at moderate speed and load will wear to its elongation limit (typically 3% for most drives) in a fraction of the time a properly lubricated chain will last. The ACA defines four lubrication types based on chain speed:

• Type I -- Manual or drip (low speed, < 500 ft/min): Oil applied manually or by drip lubricator to the link plate edges at 8-hour intervals
• Type II -- Bath or disc (medium speed, 500–2,500 ft/min): Lower strand runs through an oil bath, or a slinger disc picks up oil from a sump
• Type III -- Oil stream (high speed, 2,500–5,000 ft/min): Continuous oil stream from a pump directed at the inside of the lower strand
• Type IV -- Forced feed (very high speed, > 5,000 ft/min): Pressurized oil spray directed at all chain articulation points

The critical lubrication point is the pin-bushing interface inside the chain. Oil must penetrate between the pin and bushing where the articulation occurs. Applying grease to the outside of the chain does very little -- grease is too viscous to penetrate the pin-bushing clearance. Use a light machine oil (SAE 20 or 30 equivalent) applied to the inner side of the chain where it enters the sprockets.

Chain Wear Measurement and Replacement Criteria

Roller chains wear by elongation. As pins and bushings wear, the effective pitch increases, causing the chain to ride higher on the sprocket teeth. Eventually the chain rides over the tooth tips and jumps -- a dangerous failure mode.

Measure chain wear by spanning a known number of pitches under light tension and comparing to nominal. Standard replacement criteria per ACA:

• 3% elongation: Replace for most drives. Beyond 3%, the chain skips teeth on standard sprockets.
• 1.5% elongation: Replace for drives with large sprockets (> 67 teeth) or precision timing applications.

Example: 10 pitches of #60 chain (3/4" pitch) should measure 7.500" nominal. At 3% elongation, it measures 7.725". If you measure 7.700" or more, replace the chain.

Always replace sprockets with the chain. A new chain on worn sprockets (with hooked tooth profiles) wears the new chain 3–5 times faster than running on new sprockets. The cost of sprockets is trivial compared to unplanned downtime from a premature chain failure.

Chain tension should be adjusted to provide approximately 2% of the center distance as sag on the slack side. Too tight increases bearing loads and accelerates wear. Too loose causes whip and engagement problems.