Drive Service Factor Calculator

The service factor and the design power, from three published families at once — the chain table Renold Jeffrey and Tsubaki agree on, the RMA-style V-belt table with its idler additions, and Khurmi’s multiplicative textbook method. They disagree by up to 0.36 on ordinary duties and they disagree about SHAPE too: the chain tables have no daily hours axis at all. All three are printed, none is averaged, and the page says plainly that the manufacturer’s own figure beats every one of them.

Last updated 24 Sep 2026

Drive service factor

Driver and duty → design power

Power to transmit (kW)The power the machine needs. The factor turns it into the design power you take to a catalogue.

Which drive are you sizing?This decides which family’s factor the answer uses. All three are printed below whatever you pick, because the disagreement between them is the point of the page.

What is driving it?

Shock class of the driven machine — the chain family’s axisUsed by the chain table and by Khurmi’s K₁. The V-belt table classifies machines by name instead, which is the next field.

Driven machine group — the V-belt family’s axisThe V-belt table names the machines rather than grading the shock, so the two families are asking you slightly different questions about the same machine.

Daily running hoursThis is the axis the chain family does NOT have. Renold Jeffrey’s and Tsubaki’s chain tables have two dimensions and no hours in them at all; the V-belt table has three bands of hours; Khurmi’s method has an explicit hours factor. That is the most interesting disagreement on this page.

Idler pulley, if any — V-belt family onlyBestORQ publish additions to the service factor for an idler: nothing for one inside on the slack side, 0.1 for outside slack or inside tight, 0.2 for outside on the tight side.

Chain lubrication — Khurmi’s K₂ onlyNeither the chain nor the V-belt table has a lubrication axis. Khurmi’s method does, and it is the widest factor in his method: 0.8 for continuous lubrication against 1.5 for periodic — a ratio of nearly two.

Not a circuit: one service-factor axis with the three published families' answers to YOUR duty marked on it, so that the disagreement between them is a distance rather than three numbers in a list. The three vertical rules are the band boundaries the verdict above uses — 1.15, 1.35 and 1.6 — and the interesting thing to do is change the daily running hours and watch which markers move. The chain marker will not move at all, because Renold Jeffrey's and Tsubaki's chain tables have no daily-hours axis; the V-belt marker steps at 5 and 16 hours; Khurmi's steps at 8 and 16, somewhere else again. Each marker is gated on its own family's value and nothing else, and the axis stops at 2.0 — Khurmi's multiplicative method can reach 3.375, and a marker pinned at the right-hand end is telling you it has gone off the scale that either lookup table covers.

1.30Example

7.5 kW driving a machine tool — moderate shock, V-belt group 2, 6 to 16 hours a day — from a normal-torque electric motor, with drop lubrication and no idler

Design power is the only output that matters

P_design = P × K_s · chain: K_s from (driver, shock class) · V-belt: K_s from (machine, driver group, hours) + idler · Khurmi: K_s = K₁ × K₂ × K₃ (load × lubrication × hours)

P_design: the power you look up in a rating table. Everything else on this page exists to produce it, and a rating table looked up with the nominal power instead is the single commonest way a drive ends up undersized
chain K_s: two dimensions, nine cells, 1.0 to 1.7. Renold Jeffrey and U.S. Tsubaki publish identical numbers. No hours axis
V-belt K_s: three dimensions, twenty-four cells, 1.0 to 1.8, plus up to 0.2 for an idler. The driven machine is identified by name rather than graded for shock
K₁ × K₂ × K₃: a multiplicative method, so its range is the product of the ranges: 0.80 at the bottom and 3.375 at the top, wider at both ends than either lookup table
the idler addition: 0.0 inside slack, 0.1 outside slack or inside tight, 0.2 outside tight. A reverse bend on the loaded side costs most, which is what a fatigue argument predicts

Worked example

7.5 kW driving a machine tool — moderate shock, V-belt group 2, 6 to 16 hours a day — from a normal-torque electric motor, with drop lubrication and no idler
CHAIN family: a moderate-shock load on an electric motor is the middle cell of the first column — 1.30. The daily hours do not appear, because that table has no hours axis
V-BELT family: machine tools are in the second driven-machine group, a normal-torque motor is Group 1, and 6 to 16 hours is the normal band — 1.20. No idler, so nothing is added
KHURMI's method: K₁ = 1.25 for a variable load with mild shock, K₂ = 1.00 for drop lubrication, K₃ = 1.25 for 16 hours a day. 1.25 × 1.00 × 1.25 = 1.5625
So the three answers are 1.30, 1.20 and 1.5625: a spread of 0.3625, which is 30% of the smallest. On 7.5 kW that is 2.72 kW of design power riding on which table you opened
The answer above uses the chain family, because that is what is selected: design power = 7.5 × 1.30 = 9.750 kW. Take that to the roller chain selection calculator — not the 7.5
And the sentence that matters more than any of the numbers: a manufacturer's own factor for your specific machine class beats every table on this page. This is a first pass

The CHAIN family — Renold Jeffrey and U.S. Tsubaki, agreeing cell for cell

Two dimensions and nine cells, and two makers publish identical numbers — which is why this table is treated here as settled rather than as one publisher’s view. What is NOT in it is the thing everybody expects to find: the daily running hours. There is no hours axis. For a chain drive the hours enter through the power rating table’s own life basis, not through the service factor, so a chain drive running round the clock and one running a single shift get the same factor from this table. Note also the column order: an internal combustion engine with a HYDRAULIC drive is gentler than one with a mechanical drive and, on a moderate or heavy load, gentler than an electric motor — because the hydraulic coupling absorbs the engine’s torque ripple. A drive is rated for the duty it sees, not for the power it nominally transmits. The service factor used here is stated; the manufacturer’s own factor for your machine class and daily running hours takes precedence over any general table.
Shock class of the driven machine	Electric motor or turbine	Internal combustion engine with hydraulic drive	Internal combustion engine with mechanical drive
Uniform	1.0	1.0	1.2
Moderate	1.3	1.2	1.4
Heavy	1.5	1.4	1.7

The V-BELT family — the RMA-style table as BestORQ publish it

Three dimensions where the chain table has two: the driven machine (by name, not by shock grade), the driver group, and the daily hours. Group 1 is normal-torque AC, shunt-wound DC and multiple-cylinder engines; Group 2 is high-torque and single-phase AC, series and compound-wound DC, and single-cylinder engines. The bands are: intermittent, 3–5 hours a day or seasonal; normal, 6–16 hours a day, peak or occasional start at or below 200% of rated load; continuous, 16–24 hours a day, or a start or peak load at or above 200% of rated load. BestORQ also publish additions for an idler pulley, which no other table on this page has: inside on the slack side adds nothing, outside slack or inside tight adds 0.1, and outside on the tight side adds 0.2 — a bend that puts the belt into reverse curvature on the loaded side costs the most, which is exactly what you would expect from a fatigue argument. These dimensions come from a published standard’s table, not from a formula. The standard itself is cited below and the printed values are attributed to the catalogue they were taken from; a different publisher may round differently in the last digit.
Driven machine	Group 1 driver, intermittent	Group 1, normal	Group 1, continuous	Group 2 driver, intermittent	Group 2, normal	Group 2, continuous
Agitators for liquids, blowers and exhausters, centrifugal pumps and compressors, fans up to 10 hp, light-duty conveyors	1.0	1.1	1.2	1.1	1.2	1.3
Belt conveyors for sand or grain, mixers, fans over 10 hp, generators, line shafts, laundry machinery, machine tools, punches, presses, shears, printing machinery, positive-displacement pumps, revolving and vibrating screens	1.1	1.2	1.3	1.2	1.3	1.4
Brick machinery, bucket elevators, exciters, piston compressors, drag, pan and screw conveyors, hammer mills, paper-mill beaters, piston pumps, positive-displacement blowers, pulverisers, saw-mill and woodworking machinery, textile machinery	1.2	1.3	1.4	1.4	1.5	1.6
Gyratory, jaw and roll crushers, ball, rod and tube mills, hoists, rubber calenders, extruders and mills	1.3	1.4	1.5	1.5	1.6	1.8

KHURMI’s textbook chain method — three factors multiplied

A different SHAPE again, and the shape is the interesting part. It has an hours factor, which the chain family’s table does not. It has a LUBRICATION factor, which neither of the other two has — and which is the widest term in the whole method: 0.8 for continuous lubrication against 1.5 for periodic, a ratio of 1.875, larger than the entire range of the chain table. And because the three are multiplied rather than looked up, the extremes are wider than either published table reaches: the lowest combination is 1.0 × 0.8 × 1.0 = 0.80 and the highest is 1.5 × 1.5 × 1.5 = 3.375. No lookup table on this page goes below 1.0 or above 1.8. That is worth knowing before you use a multiplicative method and a tabular one interchangeably. The same designation can mean different dimensions in different standards families — ANSI against ISO, inch against metric, one national standard against another. The family used here is named beside every figure; check which one your part was made to.
Factor	Condition	Value
K₁ — load	Constant load	1.00
K₁ — load	Variable load with mild shock	1.25
K₁ — load	Heavy shock loads	1.50
K₂ — lubrication	Continuous lubrication	0.80
K₂ — lubrication	Drop lubrication	1.00
K₂ — lubrication	Periodic lubrication	1.50
K₃ — hours	8 hours a day	1.00
K₃ — hours	16 hours a day	1.25
K₃ — hours	Continuous service	1.50

Six duties, read three ways — the disagreement, printed rather than averaged

No column here is wrong. They are three answers to three slightly different questions, from publishers who were each sizing something specific, and averaging them would produce a number nobody published. What the spread column shows is how much the CHOICE OF TABLE is worth — and on the duties in the middle of the range it is worth more than a whole class of shock. Two structural reasons for most of it. The chain family has no hours axis, so it cannot distinguish a single-shift drive from a continuous one and the other two can. And Khurmi’s method multiplies three factors instead of looking one up, so it reaches both further down (0.80 with continuous lubrication on a constant load) and much further up (3.375) than either table. Use the family that matches the drive you are sizing, and then — every one of these sources says it — use the manufacturer’s own figure for your machine class in preference to any of them. The same designation can mean different dimensions in different standards families — ANSI against ISO, inch against metric, one national standard against another. The family used here is named beside every figure; check which one your part was made to.
Duty	Chain family	V-belt family	Khurmi’s method	Spread	Spread (%)
A centrifugal pump on a normal-torque motor, one shift	1.00	1.00	1.0000	0.0000	0.0
A machine tool on a normal-torque motor, two shifts	1.30	1.20	1.5625	0.3625	30.2
A vibrating screen on a normal-torque motor, round the clock	1.30	1.30	1.8750	0.5750	44.2
A hammer mill on a normal-torque motor, two shifts	1.50	1.30	1.8750	0.5750	44.2
A jaw crusher on a single-cylinder engine, round the clock	1.70	1.80	3.3750	1.6750	98.5
A line shaft on a multi-cylinder engine with a mechanical drive, one shift	1.20	1.10	0.8000	0.4000	50.0

Factors this page does NOT carry, and why

A service factor page is unusually easy to pad, because every plausible influence can be given a plausible coefficient. This table is the list of influences that are real and whose coefficients were not found, kept separate from the ones that were. The rule for the batch was that a figure has to come from a named document.
Factor	Status	What was found
Ambient temperature	REFUSED	No source consulted in this batch published a numeric temperature correction to a drive service factor. Temperature certainly matters — it changes the lubricant’s viscosity on a chain and the elastomer’s properties on a belt — but it appears in the literature as advice rather than as a number, and this page will not turn advice into a coefficient.
Altitude, dust, wet or corrosive service	REFUSED	Same reason. Tsubaki’s selection material discusses atmospheric conditions and does not give a factor for them.
An idler pulley	CARRIED, V-belt family only	BestORQ publish it: inside slack side 0.0, outside slack side 0.1, inside tight side 0.1, outside tight side 0.2. Neither chain source gives an idler factor, so selecting one here changes the V-belt column and leaves the other two alone — which is itself an honest representation of the state of the published tables.
More than one shift	CARRIED as the hours axis	In the V-belt family and in Khurmi’s method it IS the hours axis and needs no separate factor. In the chain family there is nothing to carry it: the table has no hours dimension. That is not an omission in this page.
Chain lubrication method	CARRIED, Khurmi’s method only	Khurmi’s K₂ — 0.8 continuous, 1.0 drop, 1.5 periodic. Neither lookup table has a lubrication axis, even though lubrication is the single biggest determinant of chain wear life.
A gear-drive (AGMA) family	NOT CARRIED	The brief for this batch asked for at least two families and this page carries three, but an AGMA enclosed-drive service factor table was not obtained to the standard the others were, so it is absent rather than approximated. Where a gearbox is in the drive train, its own maker’s service factor governs it and not this page.

Three families, three shapes, and no average

The service factor is not one table, and this page’s whole job is to stop you treating it as one. Three families were consulted here and all three are published, sourced and in current use. The chain family — Renold Jeffrey’s and U.S. Tsubaki’s, which agree cell for cell — has two dimensions and nine cells and runs 1.0 to 1.7. The V-belt family, the RMA-style table as BestORQ publish it, has three dimensions and twenty-four cells, runs 1.0 to 1.8, and adds up to 0.2 for an idler. Khurmi’s textbook chain method multiplies three factors and therefore spans 0.80 to 3.375. On the very first duty anybody would try — a machine tool on a motor, two shifts — they give 1.30, 1.20 and 1.5625. This page prints all three and never averages them, because an average of three published numbers is a fourth number that nobody stands behind.

The most interesting disagreement is about SHAPE, not about size. The chain tables have no daily-hours axis. None. A chain drive running one shift and one running round the clock get the same service factor from Renold Jeffrey and from Tsubaki, because for a chain the hours are supposed to enter through the power rating table’s own life basis instead. The V-belt table has three bands of hours. Khurmi’s method has an explicit hours factor AND a lubrication factor that neither table has — and that lubrication term, 0.8 for continuous against 1.5 for periodic, spans a ratio of 1.875, wider than the entire range of the chain table. So the three are not three estimates of one quantity. They are three different models of what makes a drive’s duty harder, and the chart above shows the consequence directly: as the daily hours climb, one line is flat and two step.

One structural warning about mixing a multiplicative method with a tabular one. Khurmi’s K₁ × K₂ × K₃ can reach 0.80 at the bottom — a constant load, continuously lubricated, eight hours a day — and 3.375 at the top. Neither lookup table on this page goes below 1.0 or above 1.8. So if you compute a factor of 2.4 from the textbook method and then look up a chain in a catalogue whose ratings were established against a table that tops out at 1.7, you are not being conservative in a way the rating data understands; you are applying two safety systems to the same risk without knowing whether they overlap. Pick a method and stay in it, and if the answer is uncomfortable, get the maker’s figure.

What the columns are actually asking you. The chain table grades the driven machine by shock class and the driver by type, and its column order rewards a close reading: an internal combustion engine with a HYDRAULIC drive comes out gentler than one with a mechanical drive and, on a moderate or heavy load, gentler than an electric motor — because a hydraulic coupling absorbs the engine’s torque ripple, which is the thing the factor is really about. The V-belt table does not grade shock at all; it names machines, in four groups, which is easier to use and harder to extrapolate from. If your machine is not in the list, the shock-class axis is the one that will generalise. And the V-belt table’s driver groups split on starting torque rather than on fuel: a high-torque or single-phase motor sits in the same group as a single-cylinder engine, which the chain table has no column for at all.

This is a first pass, and every source consulted says so in its own words. A manufacturer’s own service factor for your specific machine class beats any general table, because the maker knows what its own rating data was established against and a general table does not. Where a gearbox is in the train, its maker’s factor governs that part of it — an AGMA enclosed-drive table is deliberately absent from this page rather than approximated. And there are influences this page refuses to quantify at all: ambient temperature, altitude, dust and wet service are real and are discussed in the literature as advice rather than as coefficients, so they are listed as refused rather than invented. Once you have a design power, take it to the roller chain selection calculator for a chain or to the belt tension and shaft load calculator for a belt — where you will find that the design power is only the beginning of what a belt drive does to its bearings. The chain length and centre distance calculator, the sprocket geometry calculator and the chain wear and elongation calculator handle the rest of the drive.

Frequently asked questions

What is a service factor, in one sentence?

The number you multiply the transmitted power by before you look it up in a rating table, so that the drive is sized for the duty it sees rather than for the power it nominally carries. A rating table assumes a particular kind of load and a particular kind of driver; a service factor is how you tell it yours is worse. Looking a drive up on the nominal power is the single commonest way a drive ends up undersized.

Why do different tables give different factors for the same machine?

Because they are not the same table with different numbers — they are different models. The chain tables have two axes, the driver type and the driven machine’s shock class, and no hours. The V-belt table has three, the driven machine by name, the driver group by starting torque, and three bands of daily hours. Khurmi’s textbook method multiplies a load factor, a LUBRICATION factor and an hours factor. On a machine tool on a motor for two shifts they give 1.30, 1.20 and 1.56. None is wrong; each was written by somebody sizing something specific. The page prints all three and the spread between them.

Do the daily running hours change a chain drive’s service factor?

Not in the tables Renold Jeffrey and U.S. Tsubaki publish — there is no hours axis in either. For a chain drive the running hours are supposed to enter through the power rating table, whose ratings are established for a stated life, rather than through the service factor. So a single-shift chain drive and a continuous one get the same factor. That surprises people, and it is worth noticing what the other two families do with the same information: the V-belt table adds 0.1 to 0.2 for continuous service and Khurmi’s K₃ adds 50%. If your rating data does not state a life, the higher figures are the safer reading.

Which factor should I use if my drive has both a chain and a belt?

Each one’s own. Size the belt with the V-belt family’s factor and the chain with the chain family’s, because each factor was calibrated against the rating data for that kind of drive. Do not compute one factor and apply it to both, and do not take the higher of the two and use it everywhere — that produces a drive that is oversized in one place and no safer in the other. If a gearbox is in the train, its maker’s own service factor governs that part.

Why is an engine with a hydraulic drive gentler than an electric motor?

Because the hydraulic coupling absorbs the engine’s torque ripple, and torque ripple is what the service factor is really about. In the chain table an internal combustion engine with a hydraulic drive gets 1.0 / 1.2 / 1.4 across the three shock classes, against an electric motor’s 1.0 / 1.3 / 1.5 and a mechanically coupled engine’s 1.2 / 1.4 / 1.7. So on a moderate or heavy load the hydraulically coupled engine is the gentlest driver of the three. The same logic is why a soft starter or a fluid coupling is often better engineering than a bigger drive.

Can a service factor be less than 1?

Only in Khurmi’s method, and only through the lubrication term: K₂ = 0.8 for continuous lubrication. Neither lookup table on this page goes below 1.0. Treat that 0.8 as a statement about a properly engineered oil bath or oil stream, not about an oiler someone remembers on a Friday, and notice how much it implies: against the 1.5 for periodic lubrication, the lubrication term alone spans a ratio of 1.875, wider than the whole range of the chain table. For a chain drive, lubrication really is the single biggest determinant of wear life.

Is there a factor for ambient temperature or a dusty environment?

Not one that this page will give you. Those conditions matter — temperature changes a chain lubricant’s viscosity and a belt elastomer’s properties, and dust is abrasive — but no source consulted in this batch published a numeric correction for either. They appear as advice, and turning advice into a coefficient would be inventing a number with a decimal point on it. They are listed as refused, with the idler factor beside them as an example of an environmental effect that IS published, so you can see the difference.

Related calculators

References

Renold Jeffrey. Roller Drive Chain Selection. The source for the chain service factor table used here (uniform 1.0 / 1.0 / 1.2; moderate 1.3 / 1.2 / 1.4; heavy 1.5 / 1.4 / 1.7, by electric motor or turbine, internal combustion with hydraulic drive, and internal combustion with mechanical drive), the multiple strand factors (1, 1.7, 2.5, 3.3, 3.9, 4.6), and the drive checklist: “small sprocket should have 17 or more teeth”, centre distance “within the optimum range of 30–50 pitches”, and speed ratio “7:1 or less (optimum)”.
U.S. Tsubaki. RS Chain Drive Selection (chains.ustsubaki.com). Confirms the same service factor table and the same multiple strand factors cell for cell, from a different maker — which is why those two tables are treated here as settled rather than as one publisher’s opinion. Also prints the chain speed relation S = P·N·n/12 (inches, rpm) and the chain length and centre distance forms.
BestORQ. V-Belt Quick Selection Guide. The V-belt service factor table used here: four driven-machine groups with their named examples, driving Group 1 (normal-torque AC, shunt-wound DC, multiple-cylinder engines) against Group 2 (high-torque and single-phase AC, series and compound-wound DC, single-cylinder engines), and three service bands — intermittent 3–5 hours, normal 6–16 hours, continuous 16–24 hours or a start above 200% of rated load. Also the idler additions: inside slack side 0.0, outside slack side 0.1, inside tight side 0.1, outside tight side 0.2.
R.S. Khurmi and J.K. Gupta, A Textbook of Machine Design, chapter 21 Chain Drives and chapter 20 V-Belt and Rope Drives (the chapters as distributed by Al-Mustansiriyah University and Al-Mustaqbal University). Source for Table 21.2, factor of safety for bush roller chain against the smaller sprocket’s speed; for the service factor as K1 × K2 × K3 (load, lubrication, hours); for the recommended teeth on the smaller sprocket against velocity ratio; for the V-belt tension ratio 2.3 log(T1/T2) = μθ cosec β with β the groove HALF angle and a groove angle of 32° to 38°; for the centrifugal tension Tc = mv²; and for the maximum-power condition Tc = T/3.
MechanixCalc. ISO 606 Calculator — Roller Chain Selection & Sprocket Geometry. Prints the ISO 606 sprocket forms d = p/sin(π/z), da = p(0.6 + cot(π/z)) and df = d − d_roller, so the tip-diameter form is the same in the ISO family as in the ANSI one; and states the selection route used here — “the chain is selected by comparing the design load (nominal power × service factor) against the chain’s minimum breaking load divided by a speed-dependent safety factor”.
Diamond-Drives by Timken. Frequently Asked Questions. Maximum allowable wear elongation “approximately 3% for most industrial applications, based upon sprocket design” and “approximately 1.5%” where centres are fixed or the drive must run smoothly; the 200/N relationship, N being the teeth in the large sprocket; and minimum tooth counts against speed — 12 slow, 17 medium, 25 high. Its stated range of application for 200/N contradicts Reliable Plant’s; see the note on the wear page.
Tsubakimoto Chain. Coefficients Used in Selection, chain-guide.com 4.1.2. States the reason the multiple strand factor is not the number of strands: “the loading is unequal across the width of the chain, therefore, the transmission capability is not a direct multiple of the number of chains.”