For some time now I have been hearing of problems intrinsic to Van’s front wheel design in that there have been instances of wheel shimmy and the loss of preload on the front wheel bearings. My investigation into the problem has led me in a couple of directions.
First, there is the issue of the bearings spinning on the axle and the subsequent damage that is caused to the yoke. Van’s has tried to fix the initial problem due to a spacer that was too thin. At 1/16″, the spacer would bite into the bearing race and start to spin on the axle shaft. The spacer would then start cutting into the aluminum yoke and all bearing preload would be lost. This results in front wheel shimmy and all of its inherent risks.
Van’s tried to fix the problem with a thicker spacer made of aluminum which was 1/4″ thick. This has lead to the second problem.
The thicker spacer now contacts and preloads on the seal of the bearing which is leading to seal breakdown and the resultant spinning of the spacer, galling of the yoke and ultimately seal separation from the bearing. I have seen this personally and it is quite disconcerting.
To understand this all we need to revert back to the design philosophy of the Matco wheel that Van’s uses in many of its home built kits. This wheel and bearing set-up requires a preload to be placed against the cone so that it stays seated in the cup and bears the load of the aircraft. When pre-load is lost for whatever reason the cone can come away from the cup slightly with the ramifications being wheel shimmy, bearing damage and ultimately bearing failure.
The wheel/bearing system from Matco is designed to operate in the following manner:
The bearings are made up from two parts called the cone and the cup. The cup is pressed into the wheel hub and the cone is forced against the cup by the sleeve that is installed between it and the yoke. In this design it is very important that the sleeves that provide the pre-load on the outer steel portion of the cone be of a specific length.
The axle that supports everything has to be just a little shorter than the sum of the wheel, bearings and sleeves so that when the outer bolts are tightened the axle does not mate to the yoke. This allows the compression forces to be placed against the outer portion of the sleeves. These compression forces constitute the pre-load that forces the cone to mate properly with the cup in the hub of the wheel. When looking at the wheel while it is spinning, the seal should be stationary.
Now the sleeves have to make good contact with the steel portion of the cone so that it is forced to remain stationary. Both the steel portion of the bearing and the seal should not rotate if the system is set up correctly. The fundamental problem we are seeing with this wheel/bearing/sleeve system is that the contact with the steel portion of the bearing is not being made correctly.
When the 1/4″ thick sleeve of Van’s is used it contacts the inner 1/16″ of the seal and not the steel portion of the bearing. With the preload on the rubber surface of the seal it deteriorated quickly and the proper preload on the bearing is lost. This sleeve needs to be 3/16″ thick to do the job properly.
Compounding this problem is the fact that the steel portion of the cone that the sleeve contacts is curved. It presents a challenge in transferring the compression force from the flat face of the sleeve to the curved face of the cone. The result of this is the propensity of the cone to spin on the axle. This in turn causes the sleeve to spin on the axle and damage the yoke.
Clear as mud I know but it is important to get technical in this explanation as there are many misconceptions out there right now.
Here is how I approached fixing the problem:
First there needed to be a way of getting a consistent preload on the bearing cone without relying on precise measurements in the length of the sleeves. To do this I proposed that an adjustable sleeve on one side of the wheel and a fixed-length sleeve on the other be employed. These sleeves have a profile cut into the face that contacts the cone that matches their radii. It is cut to the proper diameter so that no part of it can touch the seal rubber.
What this does is allow the axle to yoke joint to be more robust by having a properly torqued bolt on both sides. The adjustable sleeve is both a bolt and a nut with a hole through the bolt for the axle. The head of the bolt rests against the yoke and the nut rests against the outer steel face of the bearing cone. When you loosen the nut away from the bolt it tightens against the bearing and preloads it against the bearing cone. Once you attain the proper preload you simply tighten set screws to affix its preload.
A big bonus to this is that the preload can be set without taking the wheel off. This eases maintenance and allows for quicker adjustments.
The second area I tackled is that of the spacers spinning and causing damage to the inner part of the yoke. Here I decided to have a small round milled out of the flanges of the spacers where they contact the yoke. You measure out from the center of the front wheel bolt hole and drill a hole. You then tap it for a 10-24 socket head cap screw. This screw head nests into the milled half round in the flange to keep it from rotating.
Together, these two approaches should solve the problem with the seals being damaged and the loss of preload on the bearings.
I will be offering these for sale through Cleaveland Aircraft Tools and possibly through Van’s and Aircraft Spruce in the future.
I invite your comments and I will add pictures to this post a little later.