Gears back lash

[Mac uk]

Got a Merc 20hp gearbox. nead to measure the backlash of the gears ?. I know theres a tool but how do you aplly it and use it. Should I bother with shimming and backlash or just chuck it back together and hope for the best.

 

[Laddies]

Re: Gears back lash

If the shims are returned to there proper position the case will not need reshimming, if it's a new case or the bearings and shims are not in the place they were on tear down it must be reshimed. There are no special tools for shimming, just a manual.

 

[8075mercury]

Re: Gears back lash

The way I check back lash on these 70's 20's is as follows (This is in your Seloc too):

Assemble and install forward gear and bolt pinion gear down tight. Hold the drive shaft from moving with one hand and rock the forward gear back and forth with a finger of the other hand. You need to have some minor amount of play. No play is no good. Too much is not good either.

Mercury service manual says to start with .008 to .010" forward gear shims and .015 bottom of drive shaft bearing shims. Mercury manual also says free play is .003 to .005 but do not show or tell how to measure that.

In a newer manual for a larger motor they rig up an arm clamped to the drive shaft which moves a micrometer. With the foward gear fixed, they rock the drive shaft and get a micrometer reading.

The above method works good though with the 20's and matches what you would find in a good 20 lu too.

Hope this helps. You might find a Mercury factory service manual on ebay which has all of this.

Good luck.

 

[Mac uk]

Re: Gears back lash

ive been on the Net and found out you can use a dial test indicator to measure backlash. I was thinking of mounting one on the casing and putting the tip on/against one of the prop shaft splines, moving the prop while holding the drive shaft would give me a reading. While on the Net some proffs paper mentioned diameter ratios. That made me think. The radius from the center of the prop shaft to the prop shaft spline is about 8mm, the radius from the prop center to the gear teath is 25mm. would I have to ratio any reading I got or take it as it is.

Your idea of turning the drive shaft sounds a better way to go though.

Ille try it out, Thanks