Bearings

March 5, 2001

Last month we covered case selection basics; by now most of you will have located a dead snowmobile engine or something similar, and have disassembled and cleaned it. The next (and most important) step is to decide on the right bearing to drop into the case. 

Bearing selection is not as straightforward as you might at first think. Some of the factors involved are:

• size
• load
• speed
• type
• material
• cooling
• lubrication
• price

Since we are focusing on snowmobile engines, the simplest approach is to replace the original bearings with similar types. Although it is possible to use oil-journal, air-journal and magnetic bearings, a lot more engineering is required with these types.

Most snowmobile cases are designed for a ball bearing with a bore of 30mm (millimeters), outer diameter of 62mm, and a width of about 14-16mm. Some engines are fitted with roller and ball bearing combinations, and some engines come with 72mm ball bearings at the outer positions with 62mm ball bearings at the inner. -- The Sachs case that we are using was originally fitted with 62mm ball and roller bearings.

Ball bearings are used for higher speeds and their ability to handle both radial and axial loads equally. 

Roller bearings will allow much greater radial loading, but at the cost of low axial loads and speed.

Since the life of the engine is in the bearings, we need to examine the most important factor in maintaining long life. The first and most important factor is lubrication (and cooling). Bearings are rated for their LIO life factor -- which is basically: 90% of the bearings of a particular type will spin or turn one million revolutions at the maximum specified load before "flaking". Metal flaking from the rolling elements or races occurs as the bearing begins to break down. As flaking continues, the bearing destroys itself in a short time. So the goal is to prevent flaking at all costs. 

Some of the causes of flaking are:

• elevated temperatures
• high loads
• oscillations
• hammering

The best strategy to deal with these destructive factors is: reduce the load on the bearing & use the best lubrication system possible.

Since the lubrication system is the subject of a future session we won't go into detail here, other than to say that we'll use a jet lubrication method which will reduce heat & oscillation and allow us to use low-cost bearings at 2-3 times their rated speeds.

Loading is really the main factor we need to address if we want to extend engine life. Since highly loading a bearing deforms the rolling element and causes it to flex as it turns, heat and metal fatigue lead to premature failure. Conversely, if there is no load at all, gaps between the rolling elements and the races allow oscillations & microscopic hammering to pit the surfaces -- leading to larger gaps between the elements, increased hammering and again, premature failure. The ideal situation is to have a certain amount of load (or pre-load) and stay as far under the maximum load rating as possible.

Working with a snowmobile case limits us to certain maximum outer diameters on the bearing. Our Sachs case limits us to 62mm bearings. To increase the load rating we can do three things:

• decrease the shaft diameter
• use a wider bearing
• use roller rather than ball bearings

By decreasing the shaft diameter we also decrease the load carrying capability of the shaft. Since the rotors are overhung beyond the bearings, the shaft can handle only a certain amount of rotor weight at high rotational speeds before flexing and eventually failing. So there is a trade-off between shaft loading and bearing loading.

A good design will limit the horsepower to around 30-40 hp (which in turn limits the overhung weight). Shaft diameter should come in at 30-35mm with minimal keyways to reduce shaft integrity. Bearings should be as wide as possible to allow greater loads, and may be a combination of roller and ball types to give us good radial and axial load characteristics.

The last factor to cover is price. While a good medium-speed ball bearing is priced at $12 - $20, roller types are about double that, and high-speed angular contact ball bearings are about $160 - $180 per piece. Since we are designing this engine for low cost, our approach is to use the lower-priced ball and roller types. 

A good place to start your data gathering is NTN Bearing Corporation. They provide an enormous amount of information online at www.ntnamerica.com and excellent technical manuals through their distributors. Also check out Boston Bearings, FAG, SKF, Fafnir, etc.

Next month we'll cover shaft basics, and how to start pulling it all together. Until next time -- keep the 21st century engine technologies moving forward!

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