Mouse Trap Cars: Bushing Systems
A good bushing system is one way to decrease friction and increase the performance of your mousetrap racer. Learn more about bushing systems and ball bearings.
The number one point of friction on any mousetrap car is where the axles comes in contact with the frame of the vehicle. In most cases an axle will be in direct contact with the frame and there is a lot that can be done to decrease the friction at this contact point. A bushing is nothing more than a simple bearing.
Friction at the axle points depends on:
- The diameter of the axle
- The pressure on the axle by the bearing
- The types of materials used for the axle and bearing
Larger diameter axles have greater friction when compared to the same axle of a smaller diameter. With a smaller axle diameter the force of friction is applied closer to the center of rotation and has less bite than a larger diameter axle. Try to select axles that are equal to or less than 3/16 inch in diameter.
Bearing pressure is a combination of how much a bearing is pinching on an axle and how much the weight of the vehicle is pushing down on the axle. Lighter cars will have less weight pushing down on the bearings so always select lightweight building materials when constructing your mousetrap vehicle. Also make sure that your vehicle's bushings do not pinch on the axle, try to have a small amount of play between any axle and any bushing system.
Some materials have lass friction when used together than other materials. The friction at a bearing point will depend on the materials you select for the axle and for the bushing. There are many different materials that you can match-up when making a bushing system but some times it is best to have a chart of coefficients of friction in order to help determine which of the materials that are available to use will work the best together. See bellow
Bushings are used to reduce the surface friction at an axle point. The idea with a bushing system is to match-up materials that have the less friction when used together. For example, brass rubbing on wood has more friction than brass rubbing on steel. In this example, if the brass axle can be made to rub on a steel bushing as apposed to the wood then the total friction of the axle will be reduced and performance of the mousetrap vehicle will be improved.
Bushings reduce the friction of the axle on the frame. A bushing is placed into the axle hole of the frame.
bonus tip: Bushings can be cut from brass or steel tubing but when selecting the size of the tubing to use, never select a tubing size that is an exact fit. Try to use a tubing size that is one size, or a small amount, larger than the axle diameter; bushings should allow the axle to spin freely.
bonus tip: Try to use a tubing size that is larger than the axle diameter; bushings should allow the axle to spin freely.
how to: place cut bushing into the frame or axle hole. place axle through the bushing and then lubricate.
bonus tip: Washers also make great bushing systems. As with tubing, select a washer that is one size larger than the axle so that there is a small amount of play between the washer and the axle, this will make sure that washer does not pinch the axle.
bonus tip: glue washer to the frame or over the axle hole. place axle through the washer and then lubricate.
Coefficients of Friction
There are many different materials that you can be match-up when making a bushing system but some times it is best to have a chart of coefficients of friction in order to determine which of the materials that are available to use will reduce friction the most when used together. From the table bellow you will see a common list of materials and their coefficient when combined together. The idea here is that the smaller the number the less friction two materials have when used together. Use this table as your guide to help pick the best materials to help reduce friction with your bushing system.
If available, Teflon is one of the best bushing materials you can use. Sometimes you can find Teflon washers at a local hardware store. There are many different materials you can choose from to use with a bushing system but some times it is best to have a chart of coefficients to help determine what materials will work the best when used together. From the table bellow you will see a common list of materials and their coefficient when combined together. The idea here is that the smaller the number the less friction two materials have when used together. Use this table as your guide to help pick the best materials to help reduce friction with your bushing system.
bonus tip: Bushings can be cut from plastic straws and placed into a hole in the frame.
Most bushing systems will require and additional lubricant. There are many different types of lubricants that can be used with a bushing system, some are even designed for an exact set-up so do a little research before you select a lubricant to use on your bushing system. In most cases graphite powder is an excellent lubricant but there are also many different formulas of graphite powder. We have tested many different types of graphite powder and have found that Pin Pro Graphite powder works best for most bushing set-ups.
Important: NEVER use graphite powder with ball bearings, ball bearings are already designed to reduce friction and the graphite powder will damage the bearing.
how to apply: Graphite powder can applied to the contact point between the axle and the frame. As the graphite is applied the axle/wheels are spun to work the graphite into the frame, repeat as needed.
Stabilizing a Bushing System
Busing systems need to be stabilized in order to decrease the side-to-side play. Side-to-side play occurs when the axle has extra spacing on the ends that allows for extra back-and-forth movement. Side-to-side play is not a problem unless the wheel of the vehicle is allowed to rub on the frame causing an increase in friction. Side-to-side play can be limited by using spacers or washers to take up the slack but this comes with a warning, never take up so much slack that the axle binds against the frame. Always try to have a small amount of side-to-side play in order to decrease any rubbing friction on the frame.
bonus tip: Here rubber washers are used to limit the side-to-side play. The washers are moved along the axle and closer to the frame. A metal washer is placed between the frame and the spacer in order to decrease the rubbing friction. Make sure there is some side-to-side plan and that the spacers are not pinching against the frame.
One common mistake with bushing is putting spacers to close or to tight to the frame of the vehicle. There should always be a small amount of side play between the frame and the wheels so that wheels and/or axle spacers are not pinching against the frame causing a lot of extra friction. Try to have a minimum gap of 1/8 inch between the frame and any spacers (or wheels) in order to decrease any rubbing friction. Use thrust washers and graphite powder between spacers (or wheels) and the frame to help reduce the rubbing friction.
how they work: thrust bearings are placed between the wheels and the frame to decrease the rubbing friction.
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