How to mount a load-bearing gear shaft through a wooden plate

Question

I don't have experience in mechanical engineering so I came here to ask for help. Here is sketch of one simple construction I want to realize:

Here we have 5 cm thick wooden plate, 50 cm long and 35 cm wide. I will drill a hole through the plate for 40–50 cm long shaft, which will carry a fixed gear driven by a DC motor to rotate the shaft. The shaft must be able to carry a load of at least 30 kg.

My question is, how can I "fix" this gear shaft to carry a load without falling down? Here is one shaft I found; I don't see way to use it in this situation. How would I mount a gear on this shaft?

Answer 1

The general name for what you're trying to make is a 'thrust bearing.' That means something that allows a shaft to move relative to another part, with the load being axial (down the shaft) instead of radial (perpendicular to the shaft.)

There are many types of thrust bearings, but I'm going to guess that you're looking for a simpler, lower cost solution. I'm just basing that on the fact that your plate is made out of wood.

One simple method is to use a device called a shaft collar. which is just a round clamp you can attach to your shaft to keep it from falling through the hole. Shaft collars are made in many different styles, but all operate on the same principle. It should be easy to buy one for your size of shaft.

The problem here is that if your machine runs very fast, or for more than a few minutes, the shaft collar will slowly drill through your wooden plate. To solve this, you would want to put something between the shaft collar and the wooden plate. The cheapest solution is to use a piece of plastic - UHMW is a fairly good material for this, and is pretty affordable. They also make bronze thrust washers specifically for this purpose, which are surprisingly affordable. For example, look at McMaster Carr Part #5906K513. Stacking one or even two of these between the collar and the wood would make the bearing last for a pretty long time as long as your machine isn't moving very fast.

If your machine does move very fast, you'll want to look into rolling element thrust bearings (for example, McMaster #5909K44,) which work much better and last longer, but are a bit more expensive. Rolling element bearings are also much less tolerant to abuse (dirt, dust, impact) and some require lubrication. You may also want to look at getting a shoulder machined into your shaft in that case, rather than using a shaft collar.

Don't forget to check that the wood is strong enough, 30kg of suspended load is nothing to sneeze at.

Answer 2

There are a couple issues here.

First, the shaft you found is a keyed shaft, meant to be used with something with the inverse shape on the mating hole. This isn't necessarily a bad thing, as some gears will have keys or keyways on them, to ensure that the gear doesn't rotate relative to the shaft. The alternative is sizing the bore on the gear and the shaft as a press fit, but that will be harder to assemble, and if you don't have the right tool, you risk damaging the shaft, the gear, or both. However, it allows you to use a simple round shaft, which will make the next part easier to deal with.

As far as getting the shaft to not move up and down relative to the plate, you're probably looking at some sort of flange and some bearings. If you want the shaft to rotate relative to the plate, you'll definitely want a roller bearing in there. To keep the shaft from falling through, the easiest way will be to attach a washer or something similar to the shaft. Make sure the joining method is strong enough (welding would be great, but you probably don't have access to this. Epoxy might work, but it might not be strong enough.) Then, to make sure the flange will rotate on top of the plate, use a thrust bearing between the flange and the plate.

As Ethan mentions, 30kg is a quite a bit of weight, so be sure to check that all the components in your assembly are strong enough to do their part. You'll need a good size motor to be able to turn that much weight, and you'll also want to make this stuff as carefully as possible, as poor cuts and assembling will just add to the losses in the system.