Recently I’ve been working on a prototype for a sharpening center for ShopNotes 107. Building a machine is one of my favorite shop projects. It’s always a thrill to watch turning shafts, sheaves, and moving belts in action on a tool I made myself. Experience (making poorly functioning machines) has taught me that much of the success in shopbuilt machines lies in the bearings and their installation. You won’t be proud of a machine that vibrates, or is feeble, or is inaccurate due to misaligned bearings.
There are, of course, many requirements for bearings and so there are many types. One type that I find quite handy for low rpm applications is a bronze sleeve bearing. They are easy to work with, durable, and inexpensive.
In a sleeve bearing, the load is supported through the sliding motion of one solid surface against another, very simple. The sleeve bearings you’ll find at your local hardware store are made of oil impregnated sintered bronze. Sintered bronze bearings start as a powdered bronze alloy that is molded into the correct shape and heated to fuse the bronze grains together thus creating a porous structure. Oil is then forced into the voids. As much as 18% of the bearings volume is oil and this creates a permanently lubricated bearing.
Sleeve bearings typically come in two different styles. Those that look like a short piece of tube are designed to carry radial loads, like a spinning shaft. The other style includes a flange on that piece of tube. This bearing is designed to carry both radial and thrust loads. For example, a spinning shaft that’s being pushed on from one end. The style that you choose depends upon the application.
Even though they are simple, there are a few things to keep in mind when using these bearing.
One simple way to install a sleeve bearing is to press fit it into a wooden part. However, if you plan on press fitting a bearing get the largest shell diameter available. Thin shell bearings will actually compress somewhat into a smaller internal diameter if forced into a very tight hole. The result will be a shaft that will not turn freely, if at all. (This will make you very angry.) And, always make sure to seal any wood that touches the bearing with some shellac or varnish. Remember that porous structure impregnated with oil? The oil will wick out into raw wood leaving a dry bearing on the road to failure.
Another way of mounting a bearing is to use an oversize hole with epoxy paste allowing the bearing to seat into the correct alignment with the shaft in place. I think it’s a good idea to rough up the exterior of the bearing with coarse sandpaper to allow the epoxy to get a “grip” on the bearing.
Whether you’re using epoxy to mount the bearing or even applying finish around it, protect the interior of the bearing with some wax and plug the opening. It’s hard to ream glue or paint out of the bearing without causing damage. (This will also make you angry.)
I’ve found it’s often better to build your machine around the bearings with the shaft in place. This ensures proper alignment rather than to just hope that things will turn out right down the road. Remember, there’s more to an accurate machine than precisely cut parts. Accuracy must be part of the whole process of building a tool.
The last thing to consider is the shaft. Here it pays to spend a little more and get a precision ground shaft. You can’t expect smooth performance from a piece of not quite round hot rolled stock from the hardware store.
Simple, durable, and inexpensive, a sleeve bearing is a great option for a shopmade machine tool.