Darren Reynolds, the Gears Product Manager at R.A. Rodriguez (U.K.) Limited, outlines the most important factors to consider when specifying either standard or custom worm gears sets for any type of machinery.
Worm gear sets are useful for many different types of machinery because they can offer high reduction ratios in a compact envelope. In most cases worm gears are used for reducing speed (and increasing torque), with the worm usually having a cylindrical or waisted (‘hourglass’) form with threads the same shape as the teeth on an involute rack. Without wishing to use this introduction to delve too deeply into the many options available, the worm may be left- or right-handed with single or multiple threads. In virtually every application the worm drives the worm wheel, and sometimes it is a particular advantage that the worm cannot be back-driven by the worm wheel (ie the gears are self-locking). The worm wheel is similar to a helical gear with a face that is nearly always curved to conform to the shape of the worm. For machinery, the worm is typically made of steel while the worm wheel is bronze or, for lighter-duty applications, cast iron.
Advantages of worm gear sets
In summary, the main advantages of worm gears are:
- Smooth, silent operation
- Very compact
- Good meshing effectiveness
- High speed reduction ratios possible with corresponding torque increases
Typical applications for worm gear sets
Applications for worm gear sets are extremely diverse, but the following are typical:
- Rotary tables
- Lift/elevator drives
- General-purpose right-angle gearboxes
- Steering mechanisms
What to consider when specifying standard, modified or customised worm gear sets
The following points show what should be considered but they are not intended as a comprehensive guide to specifying or designing worm gear sets.
First decide what output speed is required, as this will help in selecting the reduction ratio (see below). Bear in mind that worm gears have a maximum speed at which they can operate, and the gear specification may be different if speeds are variable rather than steady.
2. Torque ratings
In conjunction with the speed, consider what output torque is required so that the driver (typically an electric or hydraulic motor) has an adequate torque rating.
3. Reduction ratio
In many cases speed will be the primary consideration rather than torque; the output speed will therefore be the starting point when deciding on the reduction ratio, depending on the driver. If possible, select a standard reduction ratio because this will cost less than specifying a non-standard ratio and will most likely be available on a shorter lead time. Reduction ratios of up to 300:1 can be produced.
Self-locking of worm gear sets (ie the worm cannot be back-driven by applying torque to the worm wheel) depends on the lead angle, surface finish, running speed, lubrication and temperature; shock and vibration can also overcome self-locking under some circumstances. Using a self-locking worm gear set can avoid the need for other devices to prevent reverse running, but it is not advisable to rely on self-locking worm gears in safety-critical applications.
5. Centre distance
A larger centre distance will enable a larger worm wheel and worm to be used and, therefore, more power to be transmitted. As with reduction ratios, selecting a standard centre distance will help to keep down costs and lead times.
6. Left-hand or right-hand drive?
Most standard worm gear sets are manufactured as right-handed, but some left-handed sets are also produced as standard catalogue items, or custom worm gears sets can be manufactured in either hand. Typically a machine layout will take into account the use of a right-handed worm gear set but, if this is not practical, the designer should check whether suitable standard worm gears sets are available in left-handed versions, otherwise custom gears will be required.
7. Materials and finish
We have already mentioned that worms are typically manufactured from steel and worm wheels are usually either bronze or cast iron. Nevertheless, the true choice is much wider, particularly if custom gears are being specified. For example, different grades of materials can be specified for custom gears, and stainless steel is available for specialist applications. Engineers can also choose between machined or hardened and ground finishes. For light-duty applications, acetal is a popular choice for the worm wheel, in which case it is likely to be injection moulded if it is a standard catalogue item.
8. Single or double thread
Worms can have a single, double or multiple threads (often referred to as single-start, two-start or multistart worms). Increasing the number of threads increases the pitch angle and reduces the reduction ratio accordingly. There is also an impact on friction and the overall size of the gear set, so the number of starts is a fundamental consideration when specifying a worm gear set.
9. Additional features
There are a number of factors to be taken into account when deciding whether to specify standard, modified or custom gears, and the same is true for worm gear sets. However, if specifying a custom worm and worm wheel, the opportunity can be taken to incorporate other features including worm shafts, bearing journals, spigots, keyways, holes for attaching mating components or clearance undercuts.
10. Quantities required
Some machines are built as one-offs while others are manufactured in quantities ranging from tens to hundreds or thousands. For larger quantities it can be more cost-effective in the long run if custom worm gear sets are specified, as this can reduce parts count and assembly costs. On the other hand, for smaller quantities it may be more economical to design around standard gears or items that are modified by, for example, boring centres to the required diameter and/or machining keyways into the bore.
11. Delivery time
If a custom worm gear set is necessary for technical reasons, then the machine designer will have to approach potential suppliers to enquire about delivery times. Standard components are either available from stock or can be delivered relatively quickly, and modification of standard items tends to add little to the lead time. Clearly if delivery time is an issue, engineers should specify standard or modified items and check which suppliers hold stocks or can deliver within the required timescales.
A small, standard worm gear set may only cost a few pounds, whereas a larger gear set with a higher specification could cost hundreds of pounds (or even more for large custom worm gear sets). The size of the budget is likely to influence factors such as whether the gear set is standard, modified or custom, and the level of finish. Engineers should, of course, be aware that paying less for a lower-quality gear set may increase running costs (due to higher friction and reduced transmission efficiency) and higher maintenance costs due to premature wear and, possibly, unplanned machine downtime due to catastrophic failure of the worm or worm gear.
13. Backlash-adjustable or anti-backlash worm?
In high-precision applications such as machine tools, rotary tables and medical equipment, lost motion between the input and output of the gear reducer is undesirable. Using higher-precision gearing can reduce the problem of backlash, but sometimes it is necessary to specify backlash-adjustable or anti-backlash worms. One approach is to use a duplex worm in which the two flanks of the thread have slightly different modules, resulting in different lead angles and a tooth thickness that increases from one end of the worm to the other (and the corresponding gap between the flanks decreases from one end to the other). By moving the worm axially, the right thread thickness can be found to match the worm wheel tooth profile. If wear occurs (most likely in the worm wheel) the worm can be moved to present a 'thicker' portion of thread to the worn worm wheel.
An alternative method of adjusting backlash involves using eccentrics or similar arrangements to alter the centre distance by very small amounts.
Another option is to use anti-backlash worm wheels that consist of two discs sprung relative to each other to ensure full tooth engagement with the worm. An advantage of this type of anti-backlash worm wheel is that cost-effective standard sizes are available, but this design is unsuitable for heavy-duty applications.
14. What precision class is required?
As with other high-precision components such as bearings, worm gear sets are available in different precision classes. As would be expected, there is a trade-off between precision and cost, so engineers should take care to ensure that they strike the right balance. Precision classes are defined in the standards.
15. With which standard(s) should the gears comply?
There are numerous different standards with which worm gearing can comply, including the following (NB this list is not exhaustive):
- BS 721-1:1963, Specification for worm gearing. Imperial units
- BS 721-2:1983, Specification for worm gearing. Metric units
- ANSI/AGMA 2111-A98, Cylindrical Wormgearing Tolerance and Inspection Methods
- ANSI/AGMA 6034-B92, Practice for Enclosed Cylindrical Wormgear Speed Reducers and Gearmotors
- DIN 3996, Calculation of load capacity of cylindrical worm gear pairs with rectangular crossing axes
- JIS B 1723:2014, Dimensions of cylindrical worm
- ISO/TR 10828:1997, Worm gears - Geometry of worm profiles
- ISO/TR 14521:2010, Gears - Calculation of load capacity of wormgears
When deciding which standard to work with, the primary consideration is likely to be the market in which the machine incorporating the gears will be placed on the market.
Compared with other types of gear, friction levels in worm gears can be considerably higher, so it is vital to get the lubrication right in order to prevent wear and avoid excessive heat build-up. Factors that influence lubricant choice include operating temperature and gear material (for example, if a bronze worm wheel is used, then care must be taken not to use gear oils with EP additives, as the sulphur can cause significant corrosion; if plastic gears are used, the hydrocarbons in regular lubricants can react with the polymers, so non-reactive lubricants such as silicon-based lubricants are preferred). Because of the high sliding friction between the gear teeth in worm gear sets, it is important to observe the defined maintenance intervals and ensure that the correct grade of lubricant is used for replenishment each time.
17. How to support the gears
There is no point in specifying high-precision engineering components such as worm gears if the support structure is inadequate for holding the gears in the correct position relative to each other. Designers therefore need to ensure that the support structure and bearings are suitable for the loads anticipated, and remember that the worm needs axial support and appropriate thrust bearings.
18. Service factor (SF)
If the worm gear set is to achieve the required operating life, the correct service factor must be used when specifying the gears. This will depend on the number of hours per day the gears will be operating for, the number of starts per day and the type of application (moderate, heavy and extreme shock loads will require a higher service factor).
19. Vibration and/or shock loading
Vibration and shock loading will influence the service factor and also the choice of material and finish for the worm and worm wheel. Suitably specified worm wheel sets can cope admirably with shock and vibration.
20. Safety-critical applications
For applications where operation of the worm gear is either safety-critical or mission-critical, the engineer may decide to take additional measures in an attempt to avoid premature failure. The simplest way to do this is to use a service factor (SF) that is higher than the SF that might otherwise have been used. Another situation in which a higher SF might be used is if the application is new and untested, and the levels of vibration or shock loading are uncertain.
21. Thermal capacity
Because of the high sliding friction experienced by worm gears, heat generation can be problematic. Heat is transferred away from the gears by the lubricant and, in most cases, the housing dissipates this heat. If necessary, the housing can be designed with cooling fins to aid heat dissipation, or an external fan can aid cooling (often mounted directly on the worm shaft); in extreme circumstances, an oil cooler and pump can be used to remove more heat. If it is not practical to remove the heat, then it may be necessary to revise the geometry or size of the worm and worm wheel.
22. When in doubt, ask
Clearly it is a more complex task to design or specify worm gear sets than many other engineering components – and the situation is more complicated if the gears are being designed from scratch, rather than a standard set being selected from a catalogue. Fortunately there is help at hand, which can remove an element of risk from a machine building project and save valuable engineering time too. R.A. Rodriguez (UK) Ltd offers standard KHK worm gear sets through its online gear shop, as well as standard, modified and custom worm gear sets from both the German gear manufacturer Framo Morat and KHK. Customers can therefore seek advice from the gears experts at R.A. Rodriguez (UK) Ltd and, if necessary, engineers at the gear manufacturers can be consulted. If the application calls for it, R.A. Rodriguez (UK) Ltd also offers NDT (non-destructive testing) and/or full certification for all industries.