Let’s welcome back Norm Parker — our bearings specialist blogger and contributing editor of bearings-based articles. Continue watching this space for Norm’s postings and check out future issues of Power Transmission Engineering for his news-you-can-use bearings articles.
It has been a little while and I apologize for my extended absence. We have had a whirlwind of a year in the big house and between a major project and some musical chairs; 2016 has been a fairly busy and distracting year. With most of the heavy lifting with the project, I’m working on behind us, I am officially ready to get this train back on the tracks.
Though I have been offline, there has been no shortage of challenging and interesting bearing topics to discuss. Let’s begin with everyone’s favorite topic, PRELOAD!
I’m going to turn this into a full article, but I wanted to throw this out to see if anyone has had any experience with modeling differential housing preload in a somewhat flexible housing. I am working on trying to develop a true analytical model opposed to the usual method of start with some and then add more as needed.
In this application, we physically stretch the housing, insert the shims and bearings and then release the housing in hopes that we hit the correct preload – preferably the first time. The preload is verified by measuring the torque required to turn the differential with the preloaded bearings. This result is compared to a known bearing torque vs. preload relationship. In theory, our shim selection would give us the correct amount of housing stretch to achieve our desired preload. Often is the case, we find the analytical model and reality do not match and adjustments need to be made on the shop floor. What makes this problem trickier than it appears at first glance is the bearing compression vs. the housing stretch relationship and that the housing does not stretch uniformly. Keeping in mind that this entire process will take place within 0.2mm, a few microns of lost deflection results in missing the preload window.
As we dive into this problem, we will need to have accurate housing and bearing stiffness measurements – preferably analytical and physical. Usually, the shim and differential contribute negligible amounts of deflection but is good practice to include them in the initial model.
The model below is where we begin. Some interesting conversations came up as we discussed where and how the differential is fixtured. We will get into all of this and more in a full article.