MEASURING THE AXIAL CLEARANCE OF THE FREE WHEEL GEAR

Hello Edmond

This is taught in the iRMT training at the heavy maintenance level.  It is done with a dial indicator mounted to the block and the dial connected to the edge of the freewheel gear face.  The gear is pulled to a stop in one direction and the dial set to zero.  Then the gear is pushed to the extreme opposite and the difference is the clearance internally.  As the hub will always spread with each time it is removed and replaced the clearance always gets smaller.  If it is below the tolerance then with engine heat, expansion of the parts, it may seize in the housing and damage your starting motor.  (generally they will drag to explosion) 

Cheers

Edmond,

If you are going to replace the ignition housing as part of SB-912 I-028, consider having the service center where you purchase it install a new water pump shaft, seal, and drive gear. The first one of these I did, I installed these items myself, which takes a couple special tools, and I was intending on saving the old drive gear. That said, I could not press the shaft out of the gear and housing. I have a good size press, and I even cooled the shaft and rapidly heated the gear the best I could but that shaft would just not press out.  Since I already had a new shaft and seal, I just bought the new gear and discarded the old one in the housing.  After icing the new shaft and warming the new gear, the new parts went together with reasonable force. Later, for the heck of it, I tried again to get the old gear out without regard for damaging it. I used a lot force and heat and that thing would not budge.  

The next time I did this job (on another plane), I sent the old housing (with the water pump and stator still installed) to Lockwood and they sent me back the upgraded housing assembled and ready to go.  I think they charged something like 1.5 hours to do it and it’s the best money you will spend.

As RW stated, typically the freewheel axial clearance gets smaller with disassembly and reassembly as the bore in the sprag housing stretches.  It’s odd that you are out of spec in the other direction. It’s hard to imagine enough wear of the gear hub face or the sprag housing to cause this. You will be replacing the sprag housing as part of the SB so you will have the opportunity to inspect everything again. Before you pull it back apart, measure it again using a dial indicator as per RW’s instructions.  Also, be careful with the loctite used during the spray nozzle installation so you don’t clog it, and I would recommend doing the air bleed test on the nozzle to confirm once the job is complete.  

Thank you both, RW and Jeff B.  I re-performed the measurement procedure as detailed by RW with an A&P I trust. The measured movement was actually 0.045".  The primary question is:  Interpreting the wear limits in figure 9.23, is my measurement within limits? 

And, yes, Jeff B, I did get a full ignition housing with oil seals, new stator, etc.  I've already installed the sprag clutch and applied Loctite and torqued. I will perform the oil spray nozzle airflow test, then install the nozzle, retest, and re-install the ignition housing.

Edmond, this might help.  

Cheers

Edmond

I interpret the manual as saying the minimum distance is the critical one, and the wear limits apply to the minimum clearance only. More specifically the minimum clearances are:  “ New=.02”, 50% wear = .014”, and 100% wear = .01”.  “Wear” is a bit misleading in this context, it’s more accurate to say this is a declining value each time the sprag housing is removed and reinstalled… I guess that’s “wear” of a sort.

There is no Wear Limit given for the maximum clearance.  The max distance when new is .04”, and your reading of .045 would seem to me to be within the acceptable margin of error.  Probably if you removed the sprag housing and reinstalled and torqued it again you would be at the .04”. The machining to get the sprag housing to slide up the taper and stop at a precise distance is difficult, a little more or less torque may change this distance. Even the temperature of the parts when installed may change this. Also, the clamping force varies for a given torque value based on friction of the threads and nut/bolt head.  The loctite actually lubricates the threads before it hardens and locks them, so if you used only a bit of loctite on the collar nut you may have actually gotten less clamping load then if you used more. If you are uncomfortable with this variance, maybe call your Rotax Service center and pose the question. If it was me, and I was confident about the installation procedure I used, the extra .005” would be acceptable.