I listen to several podcasts, including The Amp Hour, Embedded, and the Engineering Commons. A recent (depends when you read this) Engineering Commons podcast was all about bearings. I have been wanting to write a bearings post for a while so I am using their podcast (with permission) as a basis for writing this post. Much of this post will be based on their episode, however I will also be throwing a few things into the post.
Bearing are used for supporting loads from other parts of the design. They allow mechanical elements to move relative to each other with low friction. This reduces the torque required of the motor, the power usage, and the heat. It can also help increase the life of a mechanism. For example you can put a bearing on a motor shaft for to ease rotational movement, or to allow for linear motion with a linear bearing on a screen door.
Force Loading Definitions
Radial loading – load pressing down on the shaft, most common loading on a motor shaft
Thrust loading (or axial loading) – load pressing/pulling on the head of the shaft
3 ways to reduce friction
Contact: Low friction sliding, polished surfaces, rolling elements (ball bearings), etc..
Non-Contact: Film of lubricant on the bearing surface. Hydro-static bearing is where an external source pumps a thin layer of lubricant between surfaces so they do not touch and are riding on the lubricant. These can handle high forces at low speeds. Hydro-dynamic is similar but movement of parts causes the distribution of air (and not an external pump) that reduces friction. For Hydro-dynamic to work you need to wait for the parts to spin to a certain speed to cause hydroplaning, this can cause more wear as the part is getting up to sufficient speed. Another way is with magnets, such as a Maglev train.
Flexture: Allows relative motion based on the item deforming and moving. An example would be the lid of a Tic Tac container (apparently the flavor of the Tic Tac might matter for the friction forces 🙂 ).
Full film condition bearings: Hydro-static and/or Hydro-Dynamic. As speed increases friction increases and you need more force to move the load.
Dry Condition: Loads are touching the surface and the bearing is designed to work dry
Boundary Condition: In a hybrid system the load switches from full film to dry.
See my post on oil vs grease. Oil helps remove heat (with external pump), while grease provides increased lubricity once up to speed.
3 Main Types of Bearings
Journal (or plain bearings, rotary bearing, bushing)
Solid sleeve (metal, nylon, hybrid, etc..) that has low friction surface finish on your shaft and bearing important.
Sleeves can be pressed on or have a flange. Flange can be used as a thrust bearing (axial loading). Some come with pillow blocks attached. Pillow blocks are areas with pre-drilled holes that let you mount the bearing to an existing surface.
If there is no flange then the bearing can not support thrust loading, unless you add a thrust washer or similar.
Usually the shaft is hard and the bearing material will be soft, such as a steel shaft with a bronze bearing.
PV value often used for spec’ing a bearing: Pressure (PSI) on the bearing surface X Velocity (in ft/min) of shaft. See here for more details. P x V gives a number for comparing bearings.
Many manufacturers use similar number schemes for their parts, but you need to verify.
I often refer to Journal bearings as Bushings to differentiate them from roller Bearings (which comes next).
You also should be careful of “fancy” bearings. We once had these bearings with graphite beads embedded in the surface. The problem was as the graphite wore away and chucks of graphite left the bearing it gummed up some of the mechanism which increased the forces to move that joint.
Roller Bearings (Ball or rollers)
These bearings contain a rolling element that allows the parts to move with low friction. They often cost more than the journal bearings above. They Can be better for handling impact loads, better at handling dirt, higher velocities, and shaft misalignment compared to journal bearings. Besides cost the downsides can include being heavier, rusting and being noisier.
Roller bearings often attach with a groove and a retaining ring, or a flange. Ball bearing can be purchased shielded to keep dirt out.
The retaining ring or flange can help the roller bearings provide light axial loads. You can also buy an angular contact bearing where the internal balls are riding on the sides so that the roller bearing can have low radial friction, but high axial loading to restrict side to side motion. These need to be aligned in the proper orientation for your conditions to work.
Roller bearings can often take higher loads than ball bearings, but have lower operating speeds. Rollers can also be tapered to restrict motion in a specific direction. If you use 2 tapered roller bearings they can be matched to prevent pulling axially in both directions. You can also get spherical rollers to help with misaligned shafts. Another type is needle rollers, this is where the rollers are very small diameter but a little longer to fit in tight locations.
Roller bearings are often specced with a DN value; The DN value is the average diameter (D) (inner_diameter + outer_diameter/2) X N which is the speed in RPM.
These often go back to the ways of reducing friction. You can purchase linear stages that have been designed to move in a constrained manner over a low friction surface. An example could be a part that moves on a rail.
Here are some things to think about when spec’ing your bearings
- load speed
- type of lubricant
- operating life
- how much misalignment can bearing take
- corrosive environment
- dust/dirt in environment
- hardness of shaft
- mass & volume
With Journal bearings the correct sizing between bearing and shaft is critical, especially if it is oiled filled.
With a roller bearing the force should be consistent. Generally you want a press fit on the rotating part (usually the shaft), and the outer part should stay in place.
When sizing remember surface finishes. I have seen many times where parts are sent to be anodized (fancy painting) and then the tolerances were off and the mating of the parts was not correct.
With axial loading it can be good to preload the bearing to minimize motion.
With the axial contact you just squeeze until the two pairs are touching and it should be correctly preloaded.
Sorry this post is a little more disjoined than many of my other posts, but I wanted to put this information out.
Images above from:
Main Image: http://www.bearingslimited.com/
Bushings Image: http://www.ames-powder-metal.com/
Ball Bearing Image: http://www.mscdirect.com/
Pillow Block: http://www.mscdirect.com/
Flexture Image: https://upload.wikimedia.org/wikipedia/commons/c/c7/Mint_box_polypropylene_lid.JPG
Bearing Loading Image: http://www.skf.com/
Linear Bearing Images: http://grainger.com