Must you use metal?

There are many reasons to consider plastic gears whether you’re working on a medical system, semiconductor equipment, or handheld tool.

The possibilities are limitless with moulded plastics, from orbiting compound differential gear sets, to asymmetric designs. With plastic, internal gears are no harder to make than external gears, and they can carry more load.

After versatility the next advantage of plastic gears is the physical properties of plastics themselves. Attributes like chemical resistance, lubricity, low weight, and elasticity add a whole new dimension to gears. Plastic’s elasticity means that a gear can even serve as a vibration damping element. Plastic gears can also be moulded in all sorts of colours. If a gear transmission is going to be visible, it might as well look good.

Plastics are still a relatively young technology. Material scientists are continually developing new grades, materials, additives, and fillers as well as improving consistency. Also, third-party companies that combine resins from different suppliers have begun to issue custom blends tailored specifically for gears.

Furthermore, the material cost of a moulded plastic gear is a lot less than a cut steel gear.

Other side of the coin

To the unsuspecting designer, however, the advantages of plastic gears can quickly become disadvantages – starting with its versatility and the fact that there are almost no bounds to what you can do with it. Too often the person who gets a brilliant idea has no idea how to design it, and neither do the moulders or toolmakers.

Occasionally someone will crack open a book, but standard gear design as defined in most texts doesn’t work for the typical plastic application.

Then there’s the overload associated with choice. Today’s vast number of polymers and blends, each with their own properties and shrinkages, mean that the material selection process usually turns into a wild goose chase. At best, someone will look at a spec-sheet and attempt to make assessments and selections based on properties taken out of context. Material properties in gears manifest themselves in a complex manner, requiring a more thoughtful approach.

Another pitfall has to do with tooling. There’s no shortage of opinions about gear tooling, but very little is documented. In fact, what’s presumably known about tooling and shrinkage is kept secret. There’s also a shortage of toolmakers with sophisticated inspection equipment.

I shrank the gears

The next step in plastic gear manufacture is mould development. This requires estimating shrinkage for the plastic gear geometry, a feature that has caused many potentially acceptable plastic transmissions to perform inadequately or fail.

Most of the confusion surrounding plastic gears centres on the shrinkage issue. When you design a plastic gear mould you have to allow for non-uniform and non-linear shrinkage, realising that the gear that pops out of the mould, once it cools, is going to have a significantly different shape than the cavity.

It is incorrect to presume that plastic gears shrink isotropically, or, in more common terms, like a photographic reduction. There are two aspects of plastic gear shrinkage, macroscopic and local. The body and major features of a simple symmetrical plastic gear will have one approximate shrinkage value. This includes features like the outer diameter, root diameter, base, and pitch circles.

The local shrinkage, in the area of the individual gear tooth, has a completely different shrink rate. The major effect of this different shrink is that the tooth thickness does not shrink nearly as much as other gear features. In some cases, it can actually expand from the mould due to local effects. This is most profound in unfilled crystalline materials such as acetal and nylon.

It doesn’t help that the designer or end user is usually isolated from the people who turn raw materials into finished parts. You don’t just buy a moulded gear, you buy into a convoluted and fragmented process. It’s not uncommon for a moulder, for example, to subcontract tooling to a toolmaker who’s never moulded a gear.

Although the moulder may offer some guidance, it will typically have less to do with the function of the gear and more to do with such things as mould maintenance and faster cycle times. The bottom line is that you, the designer, will have many decisions made for you without your knowledge and perhaps without your best interests in mind.

Last but not least is the trap set by production costs. Though the material cost may be insignificant, the cost of plastic gear tooling can take your breath away. Huge volumes of gears are often required to amortize the tooling investment.

Plastic problems

If a plastic gear still seems to be the solution careful considerations are still required.One of the greatest differences between a moulded plastic gear and its metal counterpart is the way it is made. Almost all metal gears are cut; plastic gears are moulded. The few metal gears that are not cut, i.e. powder metal and forged gears require very similar approaches to the ones outlined here for plastic.

Since metal gears are cut or ground to shape, they can be expected to have highly concentric features due to the turning operation. Precision diameters will not be that difficult to maintain and no shrinkage compensation is required in their manufacture.

But plastic gears are moulded and so concentricity of the bore to the tooth geometry is one of the most difficult features to maintain. While tooth geometry itself can be more precise than the average metal gear, shrinkage must always be considered and compensated for in moulded plastic. Diameter tolerances will almost always be greater for plastic gears than for metal. Plastic materials are much weaker than metal, but they also have ‘strengths’ not found in metal. Built in lubrication, ultra-light weight, low noise, and low cost, are all attributes of moulded plastic gears.

These fundamental differences confound the traditional logic for gear design and manufacture. Gear tolerances and ratings are based on metal gear construction and these standards are not ideally descriptive of plastic gear geometry.

Design calculations are based on metal material properties; they do not accurately predict plastic gear function and life. Even the plastic material properties supplied by resin vendors don’t accurately define the real material parameters of a plastic gear as it is moving in and out of mesh at high speed. Traditional plastic properties are based on long term phenomena rather than advanced fatigue.

Designing the gear

Metal gears are designed and defined with respect to their cutting process. The defined pitch circle of a metal gear describes the set-up distance with the gear to its cutting tool. Such things as addendum modification refer to additional cutting tool set-up features required to produce the desired gear shape.

The ‘whole depth’ of a gear refers to how far the cutter plunges into the gear blank. In plastics we don’t need this definition scheme and it often only causes confusion and misinterpretation. Almost all plastic spur gears these days are moulded from cavities cut with wire Electrical Discharge Machining (EDM) that can trace any 2D construction directly from CAD to machined part. Therefore, any geometry that can be represented in CAD can essentially be applied to the mould cavity.

The importance of this difference is profound. Plastic gears are not dependent on metal gear tooling to create their geometry. The gear designer is free to create the perfect mathematical gear on paper and transfer that geometry to the gear through wire EDM. One method of doing this is to let the gears design themselves through their meshing conditions. Involute geared transmissions are ideally equivalent to crossed-axis belt drives. Gear teeth cause the same rotational effect using the same path of transmission.

With these facts in hand, one can relatively size the drums per the reduction ratio of the intended gear set. Absolute size is unimportant at this stage since the final gears can be scaled to fit the intended volume. Next, the designer must select a base tooth thickness and draw the involute tooth form on one gear as well as the distance to separate the gears which will fix the working pressure angle. The outside diameter of the gear is set arbitrarily at this point. Now that one gear has been defined in the above fashion, the rest of the construction will be self-generating.

The partially constructed gear is rotated about the pitch circle of its mate, and the outline of its mate is then formed. The tip of that gear is cut off at a reasonable length and then the second gear is rotated about the pitch circle of the first to outline the root of that gear.

With this complete, the two gears are fully described at their maximum material condition. In order to account for eccentricity and moulded tolerances, the teeth can be additionally thinned, or the gears can be pulled slightly apart to allow for necessary clearance. The outside diameters can be toleranced minus from this maximum material condition to eliminate the possibility of interference.

This self-generating construction technique allows the designer to maximise the gear action for the plastic mesh. Teeth can be made longer to increase the working range of engagement or thicker to increase tooth strength. Attention must still be paid to traditional gear concerns such as contact ratio and tooth strength.

A further advantage of this generation technique is that the CAD geometry can be used to compare moulded gear features either optically or with a scanning Coordinate Measuring Machine (CMM).