Lighten Up With USA Tolerance Rings
Product Announcement from USA Tolerance Rings
Mounting bearings is the prime application for tolerance rings. They are a time tested / proven solution for solving bearing assembly and retention issues that lead to premature bearing failures in the field. Bearing manufacturers call for very tight bore tolerance control and this creates several challenges when dealing with tubing and especially with tubing in weldments. There are numerous applications in bicycles, exercise equipment, medical equipment, household furnishing, tubular frames in automotive, construction or industrial equipment.
One application is in lawn and garden equipment holding the bearings in the front wheel casters of a zero turn radius lawn mower; an application that demonstrates many of the unique advantages of Tolerance Rings mounting the bearings. The wheel caster on a lawn mower is subject to high impact forces and a rigorous environment. Overall the tube assembly, welded to the mower deck, and the mating components must be designed to handle very high external loads. This adds a good deal of bulk to the design alone, so how do you lighten up the design?
Normally, the wheel axle is supported by two top-hat plain bearings pressed into machined bores at the top and bottom of the thick wall DOM tube. The tube is welded to a steel arm that in turn is welded to the mower deck. The DOM tube is machined on both ends to the bearing manufacturers' recommended tight tolerance of .0004" or less. Often, the caster's DOM tube is machined at this stage since fixturing for turning of these long awkward attachments after welding, is not possible in many applications. The connecting arm is then welded onto the tube. In order to reduce bore distortion, due to the heat from welding, a thick tube wall section is necessary, one that is much thicker and heavier than is needed to hold the wheel axle loads. Even with the thicker tube the bore can and does show distortion resulting form the localized high heat of the weld. If this weld is performed by hand, vs. using expensive computer controlled welding, greater distortion may occur.
How can Tolerance Rings overcome the problems associated with the weld, yet and keep the design envelope the same? First, a ring is needed to hold approximately 3000 lbs of radial force, yet it needs to be flexible enough to compensate for the expected added distortion and wider bore tolerances of the thinner tubing. While keeping the tube OD the same, the tube section is significantly reduced over the length of the tube, resulting in a savings in both the weight and the cost of the tube. With the thinner, lighter tube there is bore distortion after welding of up to .006". The Tolerance Rings are able to compensate for the welding and give added tolerance for the machined bore. In many cases, the thinner section off the shelf DOM tubing sizes can be used as is for a further cost savings.
With the tolerance ring the bore diameter no longer needs to be held to less than +/- .0004 but now +/-.001" can be held easily in the DOM tubing, a savings both in machining time and scrap. In some applications, the tight machining that had been outsourced to precision shops, could now be brought back in house, giving additional savings.
With the old design it was necessary to re-bore the tube bearing bores to round them out. This is difficult because the arm is attached, making it is costly to set up each end for machining. Also there is the problem of alignment of the bores. Although not critical in a lawn mower, Tolerance Rings allow for misalignment in critical applications.
All welded steel items require some form of coating or painting. In order to protect the bearing bore tolerance it is necessary to plug or mask the bearing bore. This is a separate operation both before and after painting. There is no need to mask with a tolerance ring as the ring will compensate for the overspray that gets into the bore. At times, the assemblies can be dipped and again the variation in the bore can be taken into consideration and compensated for.
Coating or painting needs to be cured and this normally requires heat. This process in some applications results in additional distortion of the bearing bore due to stress relieving the welded assembly. The tolerance ring eliminates the need for further bore sizing or pre-stress relieving prior to painting.
We strive to work with our customers looking at the overall design constraints, the materials and the end use application to give them every advantage with a Tolerance Ring design to solve their design issues.