Friday, October 17, 2008

Geek Things I wish I had known when I was young

I suppose it is mainly Andelle and Brian who are the geeks who read this blog, so, I am mainly addressing this to them. The subject is how to make one mechanical shaft drive another one, for instance a motor to connect an actuator or, for a another instance, an engine to drive a generator. The problem that arises in doing so is that it is always impossible to make a direct rigid connection because it is impossible to perfectly align one shaft with the other. There are three main ways to solve this problem. The method chosen depends upon circumstances.

When reasonably good alignment is practical and when a compact arrangement is desired, an Oldham coupling is almost invariably used. This is an arrangement where one shaft has a crosswise slot cut into it, the other shaft has another slot cut also cut at right angles to it but perpendicular to the slot on the other shaft. A "spider" usually made of hard rubber is placed between the two facing slots. Of course the spider has a protrusion to match the slot one one side and on its other side a protrusion to match the other slot. The center of the spider is a circular disk. This type arrangement is almost invariably used for generators. As the shafts rotate, misalignment is accomodated by the protrusions sliding latterally within the grooves.

Another handy technique is to simply connect the two shafts with a spring that grips both shafts. This only works for very light duty purposes such as instruments. The shafts can be considerably misaligned, even perpendicular to each other. You can get an ordinary spring from the hardware store and epoxy the ends to each shaft (or fasten them with a screw if removability is needed, and, it usually is). Or better yet, if you have a expensive instrument, use a factory made precision coupling made by machining a hollow cylinder then machining a helix around the periphery of the hollow cylinder leaving an helix coil with rectangular crossection spring coils elements. An alternative to a metal spring coupling is to use a piece of plastic tubing between the shafts. Because plastic expands a lot with temperature, it is not a good choice for precision instruments, but it is great for a laboratory quick setup for some gimmick.

Another technique for coupling shafts is widely used in trucks for coupling the transmission output to the differential drive in the rear. In most cases there is a wide distance separation between them. This is called a Cardan's universal joint. Importantly you need TWO of them, each connecting to a third so called center drive shaft that goes between the desired input shaft and the desired output shaft. To save on weight and provide rigidity, the drive shaft in trucks is usually hollow. The center parts of each Cardan joint at both ends of the center drive shaft must be aligned with each other. A single Cardan joint is shown in


Brian said...

Something that I do when I don't want to direct couple a motor to a gear pump and there won't be a tremendous amount of torque, is to take a rubber hose that is about the diameter of the shafts and insert each shaft into it so the shafts touch in the center of the hose section. Then I take hose clamps and clamp the hose down on each shaft. I do this for two reasons... 1) if something jams in the pump, i'd rather just have the hose coupling ripped up than have my motor burn up or my gears in the pump shredded. It only costs a couple of cents to cut a new section of hose and clamp it back down once the problem is solved and 2) if the shafts are just slightly off in diameter, or not aligned 100 percent with each other, you can still fudge it and the flexibility of the rubber hose prevents the shafts from damaging each other or knocking each other out of alignment.

It obviously doesn't work in every situation, but it worked to build my 12v scooter-motor driven gear pump lift pump for my truck.

Lynnis said...

I know I am not your intended audience for this, but it is way over my head. I'm not as smart as you think I am. :)