EngineeringMechanical EngineeringPart 05 of 15
Roller Chain Drive Selection
Chain drives fail in three different ways depending on how fast they run. Read the rating chart as three overlapping failure modes rather than a single curve and the selection logic becomes obvious.
- Application and tooth factors
- Three failure modes
- Chain length in pitches
- Lubrication regime
Executive summary
Precision roller chain is a series of journal bearings held in fixed relationship by link plates. Each bearing is a hardened pin and bush on which the roller revolves; the pin and bush articulate under load, and the roller takes the impact of engagement with the sprocket tooth.
Chain is specified by pitch — the distance between adjacent bearing pin centres — together with roller diameter and the width between inner plates. Those three together are the gearing dimensions, because they determine the sprocket tooth form and width. Both British Standard (ISO 606) and ANSI chain are based on imperial pitch sizes; all other catalogue dimensions are metric.
Selecting ratio and sprockets
Sprocket tooth counts are chosen before anything else, because both the drive ratio and the tooth factor depend on them.
Nineteen teeth minimum
Aim for at least nineteen teeth on the driver sprocket. Rating data is normally published against a nineteen-tooth reference, so smaller sprockets require a correction.
Twenty-five and hardened for hard duty
Where the drive runs at high speed or takes impulsive loads, the smaller sprocket should carry at least twenty-five teeth and should be hardened.
Odd teeth, even pitches
An odd number of sprocket teeth combined with an even number of chain pitches distributes wear across the whole chain rather than concentrating it.
Upper limits
A maximum of about 114 teeth is normal practice. For large ratios, check that the angle of lap on the driver sprocket is not less than 120 degrees.
- Z1
- number of teeth on the driver sprocket
- Z2
- number of teeth on the driven sprocket
- i
- drive ratio
Application factor and tooth factor
Two multipliers convert transmitted power into the selection power used against the rating chart.
Application factor
The application factor accounts for dynamic overload arising from the combination of driver and driven machine. It is read from a grid whose axes are worth understanding directly:
| Driven machine | Smooth running driver | Slight shocks | Moderate shocks |
|---|---|---|---|
| Smooth running | Centrifugal pumps and compressors, printing machines, paper calenders, uniformly loaded conveyors, escalators, liquid agitators, rotary driers and fans. Lowest factors in the grid. | ||
| Moderate shocks | Multi-cylinder pumps and compressors, concrete mixers, non-uniformly loaded conveyors, solids agitators and mixers. Mid-range factors. | ||
| Heavy shocks | Planers, excavators, roll and ball mills, rubber processing machines, presses and shears, one- and two-cylinder pumps and compressors, drilling rigs. Highest factors, roughly double the smooth-smooth case. | ||
The driver axis runs from smooth (electric motors, steam and gas turbines, engines with hydraulic coupling) through slight shocks (six-cylinder or larger engines with mechanical coupling, motors with frequent starts) to moderate shocks (engines under six cylinders with mechanical coupling).
Tooth factor
- f1
- application factor from the grid above
- f2
- tooth factor, correcting from the nineteen-tooth reference used in the rating charts
The tooth factor exists because a smaller sprocket puts a higher tension in the chain for the same transmitted power. It is a correction to the rating basis, not a safety allowance.
Reading the rating chart
Rating charts look complicated. They are built from three simple straight lines representing three different failure mechanisms, with rounded tops where the middle mechanism takes over.
- Low speedLink plate fatigue. Exceed the recommended power here and the plates fail in fatigue.
- Mid speedBush and roller fatigue. This mechanism produces the rounded tops on each selection curve.
- High speedPin galling, caused by breakdown of the boundary lubrication film between pin and bush.
Select the smallest pitch of simplex chain that will transmit the selection power at the driver sprocket speed. That normally produces the most economical drive. Where the selection power exceeds the simplex capability at that pitch, move to a duplex or triplex chain of the same pitch rather than jumping to a coarser pitch.
When a chain has been correctly selected, the failure mode over a very long period is wear, not fracture. The best available indicator of wear performance is the bearing pressure between pin and bush — working load divided by bearing area. High pressure at high chain velocity is the combination that shortens life, and it is why lubrication method is part of the selection rather than an afterthought.
Chain length and centre distance
- L
- chain length in pitches
- C
- contemplated centre distance, mm
- p
- chain pitch, mm
- Round up to an even number of pitchesOdd pitch counts require a cranked link, which is not recommended.
- Add two pitches for a jockey sprocketWhere a tensioner is fitted for adjustment purposes.
- Aim for 30 to 50 pitches of centre distanceFor 1.5 inch pitch chain that is roughly 1524 mm at forty pitches.
- Recalculate the exact centre distanceThe actual centre distance for the rounded chain length will generally be greater than the value first contemplated.
An even number of pitches allows the chain to close with a standard connecting link. An odd count forces a cranked link, which is weaker than the surrounding chain and becomes the governing element in the drive.
Selection procedure and lubrication
- Gather the essential dataPower in kilowatts, driving and driven shaft speeds, drive characteristics, and centre distance.
- Select ratio and sprocketsNineteen teeth minimum on the driver, odd tooth counts preferred, angle of lap checked for large ratios.
- Establish the application factorFrom the driver and driven machine characteristics.
- Calculate the tooth factor19 divided by the driver tooth count.
- Calculate selection powerPower multiplied by both factors.
- Select the chain from the rating chartSmallest pitch of simplex chain that carries the selection power at the driver speed; move to multiplex before increasing pitch.
- Calculate chain length in pitchesRound up to an even number.
- Recalculate exact centre distanceFor the chain length actually specified.
- Choose the lubrication methodDetermined by chain speed and pitch — manual, drip, oil bath, disc slinger or forced circulation as speed rises.
Chain selected by this method, correctly installed, adequately lubricated, kept clean and loaded within its design capacity is typically rated for a service life in the order of 15 000 hours. Every one of those conditions is a condition, not a description — the number does not survive a drive that runs dry.
Selection checklist
- Chain standard selected deliberately — British Standard or ANSI — and stated in the specification.
- Driver sprocket at nineteen teeth or more, or twenty-five and hardened for high speed or impulsive duty.
- Odd sprocket tooth count paired with an even chain pitch count where practicable.
- Angle of lap on the driver sprocket at least 120 degrees for large ratios.
- Application factor selected from both driver and driven machine characteristics.
- Tooth factor applied as a rating-basis correction, not confused with a safety factor.
- Smallest suitable pitch selected, with multiplex chain preferred over a coarser pitch.
- Chain length rounded up to an even number of pitches, plus two if a jockey sprocket is fitted.
- Exact centre distance recalculated and within the 30 to 50 pitch guideline.
- Lubrication method matched to chain speed and specified with the drive.
- All rating chart values re-verified against current manufacturer data for the chain brand actually purchased.