Redrive - Bearing Replacement
By: Ralph Shultz
Has your Challenger redrive been bothering you lately? Perhaps you are wondering if the bearings may be failing or are about to fail? You have tried to determine if they are running rough but cannot decide either way if they are still good or not. You really would like to answer that question but the only way you see to do that is to open up the drive, remove the bearings and do a hands-on inspection. But the problem is you don't exactly know what is inside the drive or what to expect in doing that.
This article will explain what you need to know to handle the bearing change with ease. The following diagrams depict the two different redrives that are in service on Challengers today and should go a long way in relieving any uncertain feelings you might have about tackling a bearing replacement job.
Description of the Redrive Interior Components:
By referring to the diagrams above, you will see that the two designs are very similar. Note that the prop mounting flange is directly below the propeller. This flange is attached to the upper “driven” pulley by six socket head cap screws and up to this point the drives are identical. In the Hegar drive, you find the bearing shaft, a nut & washer and a snap ring. In the QCU set-up you see the bearing mounting shaft end and a snap ring. In both drives the bearing stack shows up next and is the real focus of this article. The Hegar stack consists of three ball bearings and a bearing spacer followed by another snap ring. A bearing spacer and a couple of shims complete the stack. The drive is mounted to the tower and held in place by the “Big Bad Nut” with a cotter pin locking the nut in place. In the QCU drive you find four ball bearings with no bearing spacer. Another snap ring holds the stack in. A shaft collar is mounted nesting against the inner race of the stack. This collar and the washer and the dreaded “BIG BAD NUT” end up mounting the drive in the mounting plate (tower). A cotter pin then locks and holds everything together.
Be sure to note and record the bearing numbers on your removed bearings. They will probably be marked with something like “IKS 9R-16, R-16 2RS, 2RS 16” or some similar numbers. The bearing's basic dimensions are 2.0000” O.D. X 1.0000” I.D. X .4985” width. These bearings are considered light duty bearings, but believe me, the service loads they see are anything but light duty. Supporting a working airplane propeller is a very tough job for shafts and bearings! Bottom line is that you should use good, well lubricated and sealed bearings.
While talking about the bearings you need to be forewarned that the Chinese manufactured bearings of this size have been reported by some as not holding up very well in our redrives. Use them if you must but keep a keen eye on the installation and future operations.
Taking notice of the used bearing numbers, you will probably see the sequence of -RS or -2RS included. For most manufacturers these designations refer to bearings, which have integral internal rubber seals, on one side only or on both sides respectively.
If a bearing shows a -D or -2D, the seals are made of metal. The letter Z stands for metal shields. It may be possible to find “-2RS2Z” bearings, but they are rare. The sealed bearing types are lubed and sealed at the factory, thus ensuring maximum cleanliness and lubrication throughout its service life.
Do not try to clean the interiors of these bearings in hopes of returning them to service. If a bearing checks out OK and you believe it is indeed serviceable, just wipe off the outer surfaces and reinstall it. As a matter of fact, trying to clean any bearing with a solvent is a bad idea unless you happen to have a special sonic solvent vibration tank in which to suspend the bearing in. Otherwise, it's better just to push out the old lubricant by machine packing with clean lubricant. This however is not possible on sealed bearings. A precautionary note is in order here: It is a fact that it is not good practice to mix new with used bearings. Doing this usually results in premature bearing failures.
I would think the ideal bearings for our drive application would be two -2RSZ bearings in the outer positions with the shields facing outward and one or two -2RS bearing(s) in between these bearings. However, good luck finding those -2RSZ bearings! If these are not available, try to match what was previously installed in your unit.
The following bearings will probably work as well:
In any case, try to find self-greased and sealed bearings, as they will keep the dirt and grime out of the bearing and the grease in. If sealed bearings cannot be found, applying a piece of tape over the hole in the propeller mounting flange can create somewhat of a better seal to the cavity where the bearings live. The tape might be a good idea in any case. Also, if non-sealed bearings are used, be sure they are properly “packed” with bearing grease and add some grease to the bearing cavity as well.
Re-assembly and Installation:
Note: I see no reason why a fourth bearing or two bearing spacers seated against the inner and outer bearing races, could not be used in place of the single inner bearing spacer. If this were to be done it should simplify the installation of the bearings by making it possible to press them onto the shaft and then press the bearing/shaft assembly into the pulley bore.
A few words of caution are in order at this point in the bearing(s) installation. The bearing(s) are difficult to start straight and squarely in the bore. If they enter being cocked they will bind and if pressed farther in like that they may be damaged and in all probability they WILL damage the bore beyond the acceptable tolerances for the bearing bore. At the very first indication that they may be cocked STOP and investigate. If things do not look right, turn around and press the bearing back out. Be very careful getting the remaining bearings pressed in because if they get cocked they are very difficult to press back out as another bearing may be blocking access to the side necessary for their removal. Working through a bearing's I.D. (bore) is not fun or easy!
This is what a few Challenger owners have had to say about the removal and installation of the redrive bearings.
I use a hammer and a wooden block to start it slightly, and then use the hammer around the circumference of the bearing a bit at a time until the bearing is into the bore somewhat and is straight. Takes a bit of tapping and looking and tapping and looking. I then carefully press it on in.
Hey guys-here is a little trick to make those bearings come out much easier. Put the whole unit in the deep freeze (or just outside overnight in South Dakota this time of year). Then warm up the hub (careful not too hot) with a propane torch. This method makes the bearings smaller (cold) and the hub larger (warm). You will be surprised what a difference it makes. First time I did this was in a press. The second time I did it with the cold/warm method I didn't even need a press. I just tapped 'em out with a hammer on the shaft.
Be careful when you press them out of the aluminum redrive hub so as to not damage the snap ring groove. It takes a lot of pressure as you are pressing all four bearings through the hub at once. The aluminum prop hub must be supported equally all the way around at the center or the pressure can collapse the snap ring groove that holds the bearings in place. I would not support the aluminum hub on the perimeter as you might run the risk of warping it. I would suggest that you shop for replacement bearings at a bearings specialty house such as Motion Industries or Bearing Chain Supply, depending on where you are. You might ask them the difference between the SS vs RS designation. I WOULD NOT replace them with any bearings that say "Made in China or Made In USSR!
Another way the bearing installation might be approached is to place the bearings in a box and heat them overnight with a 100-watt light bulb. While this is happening the shaft could be chilling down in a freezer. After removing the bearings and the shaft, lightly oil the shaft and install the bearings on it. This should take very little force; however check that the bearings are seated against each other. After this has been accomplished return the bearing/shaft assembly to the freezer and place the pulley in an oven set to 150 degrees F for about 2 hrs. Retrieve the shaft/bearing assembly and pulley, lightly oil the pulley bore and install the shaft assembly into the pulley seating the bearings against the snap ring. This also should take very little force.
At this point you will have completed the bearing replacement. All that is left is to install the propeller and check its tracking. Cross torque the mounting bolts to 12 - 15 ft/lbs (144 - 180 in/lbs) and safety wire them together in pairs. You may want to read the prop article located at: www.challengers101.com/PropBal.html Give yourself a pat on the back - you did it! This is now a job you know all about and hopefully do not fear. If or when there is a next time it will go faster and easier and you can take the savings to the bank.
A final thought on the service and life expectancy of the bearings. Bearing life is usually rated by expressing that life in terms of millions of rotations at a given effective bearing load among other factors. Just as an exercise let's assume the life is rated at 100 million. At our cruising flight prop speed of roughly 2700 RPM that equates to about 100,000,000 / (2700 X 60) which renders about 617 hrs. It should be noted that this would represent a bearing operating ideally. Which is probably not what our drive bearings are subjected to considering the props dynamic flight loads and varying RPMs. Taking it one step further, my guess is that a duty cycle on the (short) Hager (three bearings) drive is actually between 2 to 3 times as harsh as this and should be applied, thus reducing the expected life to say 617 / 2.5 = 247 hrs! The four bearing QCU (tall) drive is probably in the area of 1.7 and therefore might be expected to provide 617 / 1.7 = 363 hours.
These are just my personal estimates of course. Real world bearing life calculations are much more complicated than the simplified analysis presented above. However, these results do give you a sense of what might be an expected service life from our bearings.