EngineeringMechanical EngineeringPart 07 of 15
Worm Gearboxes and Geared Motor Units
Worm gearboxes are selected twice: once against the mechanical rating and once against the thermal rating. A unit that passes the first and fails the second will run hot, thin its oil and destroy its own gear set.
- Mechanical vs thermal
- Nominal vs actual ratio
- Overhung load check
- Geared motor units
Executive summary
A worm reducer converts a high-speed, low-torque input into a low-speed, high-torque output in a single compact stage. Single reduction units cover ratios of roughly 5 to 70; double reduction units extend the range far beyond that. Configurations include underdriven, overdriven, shaft-mounted, vertical and agitator arrangements.
Two ratings govern the selection. The mechanical rating is the torque or power the gear set can transmit. The thermal rating is the power the unit can dissipate continuously without the oil overheating. For continuously running single-reduction units the thermal rating often governs, and it is the check most frequently skipped.
Ratio: nominal and actual
Catalogue ratios are nominal. The actual ratio produced by the gear set is frequently a little different, and output speed must be calculated from the actual ratio, not the nominal one.
With a directly coupled electric motor, little error results from using the synchronous speeds for interpolation: 1500 rev/min for four pole (actual around 1450), 1000 rev/min for six pole (around 960), and 750 rev/min for eight pole (around 720).
Selection procedure
- Establish the mechanical dataDriver type, design torque, power and speed; driven machinery type, design torque, power and output speed with any tolerance band; duration of service and hours per day; and the maximum ambient air temperature around the gearbox.
- Calculate the required reduction ratioInput speed divided by output speed.
- Select the nearest nominal ratioIf the required ratio exceeds about 70, a double reduction unit is needed.
- Calculate nominal output speedInput speed divided by nominal ratio.
- Classify the loadSteady, medium impulsive or highly impulsive.
- Determine the service factorFrom load classification and duty hours.
- Calculate the selection capacityDesign input power multiplied by the service factor if working from the input side, or design output torque multiplied by the service factor if working from the output side.
- Make a preliminary selectionSmallest gearbox at the nominal ratio and relevant input speed with capacity above the selection capacity.
- Obtain the actual ratioFor the unit selected, not the nominal ratio.
- Check the output speedInput speed divided by actual ratio, confirmed inside the required tolerance band.
- Apply the thermal checkFor single reduction units running continuously, or intermittently without time for the oil to cool between periods.
- Check the overhung loadWhere a gear, pulley, sprocket or flywheel is mounted directly on the output shaft.
If either the thermal check or the overhung load check forces a larger gearbox, the actual ratio may change — which means output speed and torque must both be recalculated and the tolerance check repeated. The procedure loops; it does not run straight through.
Mechanical against thermal rating
Mechanical rating
What the gear teeth, shafts and bearings can carry. Determined by the service factor applied to design power or torque. Governs on intermittent duty, and on any duty where the unit has time to cool between operating periods.
Thermal rating
What the housing can dissipate without the oil overheating. Determined by a thermal service factor applied to design power or torque, then compared against the unit's thermal capacity. Governs on continuous duty, and is strongly affected by ambient temperature.
Where the thermal check fails, the options are a larger unit, synthetic lubricant, or auxiliary cooling. Ambient temperature does not affect the selection when the gearbox runs intermittently with sufficient cooling time between operating periods — but establishing that "sufficient" is genuinely true is part of the engineer's job, not the catalogue's.
The overhung load check
Any gear, pulley, chain wheel or flywheel mounted directly on the output shaft imposes a radial load on the output bearing. The most accurate route is to calculate the actual chain or belt tensions, or the gear tooth forces. Where those are not yet known, an approximate value serves for the check.
- F
- overhung load, N
- T
- output shaft torque, Nm — the design value, not the selection value
- d
- pitch circle diameter of the pulley, sprocket or gear, m
- f
- drive application factor: 1.0 chain drive or toothed belt, 1.25 gear drive, 1.5 vee belt, 2.0 flat friction belt
Where the overhung load is excessive, upsizing the gearbox is not the only answer. An intermediate layshaft carried in its own bearings and connected to the gearbox by a flexible coupling keeps the overhung load out of the gearbox entirely — often a cheaper and more maintainable solution than a larger reducer.
Geared motor units
A geared motor unit is a worm reducer supplied with an integral motor. They occupy the low-power end of the range — broadly 0.1 to 4 kW — and the selection procedure is considerably shorter than for a free-standing gearbox.
Fixed combinations
A handful of gearbox sizes, each offered with a range of nominal ratios and a range of motor frames, produces a large matrix of power and output speed combinations from a single selection page.
Four pole input
Units are commonly fitted with four pole motors at a nominal 1400 to 1420 rev/min, giving output speeds from roughly 20 to 290 rev/min across the ratio range.
Shaft or bored bush
Power is taken either from an output shaft extension or from a bored bush with key, depending on whether the unit is shaft mounted or foot mounted.
Drive classification
Four classes from light through to extra heavy duty, derived from load classification and hours per day, each with its own selection table.
- Establish the mechanical dataDriven machinery type, design torque, power and output speed with tolerance, duration of service and hours per day.
- Classify the loadSteady, medium impulsive or highly impulsive.
- Determine the drive classificationFrom load classification and hours per day, giving a class from 1 to 4.
- Select from the table for that classAgainst required output speed and power or torque.
- Check the overhung loadUsing the same expression as for a free-standing gearbox.
If the gearbox is bought as a free-standing unit and coupled to a motor of the designer's own choosing, the geared motor unit tables no longer apply and the full gearbox selection procedure — including the thermal check — must be used instead.
Selection checklist
- Required ratio calculated and the nearest nominal ratio selected.
- Double reduction specified where the ratio exceeds the single reduction range.
- Load classified and the service factor applied to power or torque consistently.
- Actual ratio — not nominal — used to calculate output speed.
- Output speed confirmed inside the required tolerance band.
- Thermal rating checked for continuous duty single reduction units.
- Maximum ambient temperature considered in the thermal check.
- Overhung load calculated and compared against the allowable value.
- Any ratio change from an upsized unit fed back through the speed and torque calculations.
- Lubricant type consistent with the rating basis actually used.
- Mounting configuration, shaft orientation and output form specified.