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Tech Info - Gears

steel spur gearsGears provide one of the most compact ways of building a speed reduction transmission between two shafts and it is this compactness that makes them attractive for our kind of designs. All my geared designs use simple parallel shafts and what are called straight spur gears. I use steel spur gears because they are readily available in (almost) standard sizes and are the least expensive, although their cost can vary quite a bit depending upon where you get them.

The "standard" bit is the gear tooth form & size and the number of teeth on the gear, for example several manufacturers can supply, say, a 1 module, 24 tooth spur gear (the "module", or "diametral pitch" for imperial gears, is the specification of the tooth size, the number of teeth is - well, the number of teeth on the circumference) however the face width of the gear, the boss diameter, the bore size etc will all probably vary between makers, as may the steel used and its strength and surface hardness and the degree of precision to which the gear is cut. Fortunately my designs don't push these steel gears to the limits and fairly bog-standard materials and accuracies are fine so I have found that even the least expensive gears will run and last. The issue is obtaining the right basic spec (module & no. of teeth) and checking that the particular gear dimensions fit the space available.

meshed straight spur gears - setting center distanceMost of the time I use 1.5 module gears, I have found that these offer a good compromise between availability, size range available and tolerance to the inevitable slight inaccuracies of positioning that arise with home built projects. Not coincidentally they are available from a number of sources.

There are several points to bear in mind when trying to use gears. They have to be torque-fixed to the shaft supporting them and throughout our designs we have used adhesive bonding methods to do this - see the gearbox plans. This has been a very successful and convenient means of attachment; other than drilling out the gear bore to the required diameter there is no other special machining required on either the gear or its shaft, the resulting torque joint is strong enough for my designs and it can also be dismantled by heating the joint.

spur gear bonded to shaftDownsides for the builder are the relative accuracy with which the shafts must be positioned for the gears to mesh properly and, off course, the cost of the gears. Accurate shaft positioning is something we deal with in our gearbox designs so have a look at the gear box plans to see how its done. If you are not convinced that gluing a gear to a steel shaft will be strong enough - try it! Even if you use domestic superglue to do the job (not what we recommend) removing the bonded gear from the shaft by brute force is very difficult - so long as the bond area is adequate (used "bossed" gears) and the bond gap and adhesive distribution is good (get the hole/shaft clearances right) the joint strength can be considerable.

I will no doubt need to use bevel gears at some stage to connect shafts at right angles but I don't think there will be a need for helical teeth on either the spurs or these bevels. The increased power transmission capacity offered by these forms doesn't warrant the extra cost for my types of designs.


 

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