Dail indicator placement

Have you ever tryed this in actual practice? This one's got my thinking a little goofy (Or should I say even more goofy than usual.

I've got a Dorf 9" rear end in the back of the shop I'll get out and check this out in the next day or 2. You had me swayed to your way of thinking, but the more I think about this, I've got to check it out in actual practice!

Anybody else that's set up a rear end lately have a comment on this?

Pretend for a moment there are no differential gears inside the ring gear, and that a solid shaft passes throught the center of the ring gear and out the side of the axle, and has a BIG degree wheel attached (let's say 24").

I'm sure we can agree that with a given amount of gear lash, for this discussion, assume 0.010", when you rock the ring gear back and forth the amount of the gear lash you will get a certain reading in degrees on the large degree wheel that is an inducation of the amount the central axis of the ring gear rotates for a given amount of gear lash.

So now, we have converted the thousandths of an inch of gear lash to degrees of rotation of the ring gear, with the degree wheel moving with it.

There is no way to explain away the fact that for a given rotation of the ring gear in degrees, which we have determined by experiment, based on the gear lash, the reading of a dial indicator will vary depending on the radius from the central axis of the ring gear/degree wheel to the point read by the dial indicator. Near the central axis of the degree wheel/ring gear, there would be only a tiny reading variation on the dial indicator, and a large (in this case, 0.010") at the outside diameter of the ring gear.

Obviously, if you were to put a pin in the outside diameter of the 24" degree wheel, you would get a reading way in excess of the 0.010 you are reading at the lesser diameter of the actual ring gear.

Therefore, it DOES mater where on the diameter of the ring gear you check the gear lash at with a dial indicator, and this is why MOST manufacturer's instructions say to read the gear lash at the outer area of the gear tooth. In this way, the manufacturer's desired spec's can be duplicated "in the field", because everyone is using the same standard measuring method.

To reduce clearance, you would move the gears closer together and visa versa, so the two cone illustration makes sense. What you would really be doing here is to set the tooth clearance by checking the end play and establishing the ratio of one to the other.

On the other hand, if you are setting the indicator on the diameter and rocking the gear back and forth, then it would matter where on the diameter the indicator is set because as the diameter increases, the radial distance from one degree of the cicle to another also increases. I believe it's normal to check at the pitch diameter. Another thing to remember with the cones is that the equalateral triangles (sides of the cone) touch each other at 45 degree angles to their axes. Because of that, when you move one of the cones perpendicular to the other, the distance between them will not increase by a 1:1 ratio to the straight line distance the axis is moved, but will increase by the ratio of .70711 times that distance. Does this make any sense at all or did I get up too early this morning?

On straight cut gears, I thought the backlash was checked from the center of the gear mesh, or you used wire.

I've checked numerous straight cut gears where the manfacturer tells you told bolt a xx" flat bar to the bolt pattern, and you check backlash so many inches down the bar. That is for repeatability on semi-critical clearances. Waukesha does this on their 4000HP and up engines.

The shape of the teeth is for several purposes, (as was taught to me in automotive school 25 years ago). The pinion is offset from the centerline (radius) of the ring gear to "lower" the driveshaft clearance to the body of the vehicle (and therefore cannot be shaped to straight radial tooth). Furthermore the teeth are helical shaped not to maintain a constant clearance across the face of the matching teeth (the outer edges are gapped farther apart than the centers of the teeth). The teeth are designed to "slide" during contact (the face of the pinion tooth "slides" in correlation to the ring gear tooth. This causes an eliptical pattern on the center of the face of the teeth (as witnessed by the usage of "white lead" or prussian blue). The teeth of each component does not match it's counterpart in entirity (full face) at any point of the rotation. The point of each tooth's contact to its counterpart, (yes, I realize that the teeth do not really touch, because of the lubrication or they would wear out fast), moves as the gears rotate. This sliding action "reduces gear chatter." Straight teeth tend to have more inherent noise.

This all said, the measurement of the backlash is a rotational measurement of the ring gear. Not a sliding action of the ring in the carrier bearings. The ring (and carrier) are not pried from side to side in order to measure the side play of that particular gear (thus testing for looseness in the carrier bearings). The backlash adjustment is a measurement of a circular distance (travel) at a given radius frm the centerline of the axle. As you get further from the axle centerline... the "distance" for a given measure of "degree" of rotation increases. (i.e. Cut a pie into 8 pieces. Take one piece of that pie and measure the distance side to side across the face of the pie at 1" from the pointed end. Then move to the outer crust of the pie and measure across the distance side to side. Though the degree of angle did not change, the distance was increased) As you move out on the radius of the ring gear the backlash ditance of rotation will increase.

Even though there is a given degree of rotation by the ring gear (though quite minute) between either "stop" (from one set of teeth touching to the next set touching, which is what you are measuring). As you move outwardly from the centerline of that measured degree (from the centerline of the ring gear)... you increase the distance between the sides of that degree angle.

Remember, you are not measuring the pre-load of the carrier bearings (side to side movement). The ring gear does not move side to side, it moves rotationally and therefore the measurement of movement around the circumfrence increases as you move outwardly on the radius of the ring gear.

If it were possible that the dial indicator reading stayed the same as the point of measurement moved closer to, or farther away from the center of the gear, I'll bet you could do some neat stuff with that setup, think perpetual motion, or time travel. I was almost convinced it was possible the lash measurement stayed the same, and the more I thought about it, the more I realized that theory was impossible. Basic geometry, as you desribe using the paper plate, says the measurement will vary, depending on how far from the axis it is taken, and I see no way to explain that the basic laws of the universe apply differently to an axle ring gear than they do to a paper plate!

I'm still going to set up the Dorf 9" third member I have on the bench as soon as I get a free moment, and verify actual measurements. Come to think about it, I have a large Eaton truck rear that I will be setting up soon. Being so much bigger, the measurements off of the inside and outside of the teeth should vary to a greater degree.