Installing rotary encoders

rtr@...
This procedure isn't difficult, but must be understood to successfully
install a rotary encoder where none have been before.

The end of a screw usually has a centerdrilled hole, the screw is usually
hardened , but the core is quite soft to take shock. A good way to do this
is to drill the appropriate pin size ( a hardened drill blank works well)
while the lead or ball screw is assembled in the machine. Don't use a
carbide bit, because if it shatters, you are in deep trouble. Try a slight
interference fit first, It won't come out that way when you use a hand
electric drill. Try for at least a one inch depth. If the screw has
enough metal sticking beyond the bearing, you can go for 1 and a half or
two inches. This means the pin will tend to cock less in the hole.
Obviously, if there isn't going to be much wall thickness left in the
screw, make an adapter pin so you can use a smaller diameter hole and still
have the same sized pin end at the encoder as the encoder shaft.

Then drill and tap for a couple of set screws at 90 degrees to allow you to
indicate it in. Put the indicator near the end of the pin, because that's
where the coupling will fit. The pin doesn't needto stick out more that an
inch when you finish. You can cut it off and file the end when you have it
in place, indicated, and clamped with the set screws. Do thiswork on the
pin gently then indicate it again. Encoders don't like a lot of vibration
so bring it within 3 thousandths or better. I'm always happier if it
comes within one and a half thousandths.

You can connect it with an oldham coupler or similar low inertia couplers.
Surprisingly, if you mount it so the shaft pin and the encoder pin come
within a few thousandths beyond the slop in the system, you can use a piece
of surgical tubing for the coupling. You can add an outer layer of heat
shrink tubing if you think the surgical tubing is too compliant, but I've
never had to. The extra stiffness will transmit more vibration to the
encoder, a bad trade off. There is little friction in good encoder
bearings so the surgical tubing wall thickness is adaquate for a coupling.
Obviously, you can't do this if you need to drive the screw or brake it
through the encoder shaft, a bad idea in any case.

Besides eccentricity in the rotation of the encoder coupling pin, the error
that kills the most encoders is failure to allow enough room between the
ends of these two shafts. All sorts of distortions occur when an axis is
stopped after a rapid movement. Ten thousandths of longitudional slop is
not unusual in a ball screw, more in an acme screw. I take a leather or
wooden mallet or a carefully handled babbit hammer and carefully tap the
screw longetudinaly toward the encoder to get some idea of how much
longitudinal slop must be allowed for. Do this on the other end of the
screw, driving it toward the end with the pin. You aren't worried about
how much it retreats from the encoder. If you can't get to the other end
of the screw, clamp something to an accessible part of the slide and tap it
toward the encoder end from there.

Once you have some feel for this longitudinal travel, use shim stock to set
the dial indicator away from the end of the pin by a your estimated amount
and make sure that the same taps do not show up on the dial indicator. If
they do, increase the pin to encoder shaft clearance.

You need to indicate the encoder mount so its shaft is concentric with the
pin you just installed. Sometimes its easier to leave the pin you
installed full length until you have indicated in the encoder mount. It's
a good idea to drill and ream for taper pins when you have the encoder
mount indicated in. If you have a choice, countersink the encoder mount so
the encoder fits snugly, and use servo clamps if the encoder will accept
them. Clamp the indicator to the pin and indicate the countersunk encoder
locating hole wall.

The last trick works for feeling eccentricity here and other places you may
want to check for it. CAREFULLY rest a screwdriver blade on the pin, then
on the encoder shaft so the screwdriver blade is pulled away from your hand
when the shaft is rotated, and so it won't interfere with your coupling.
If, when you are moving the axis rapidly, you feel a bounce on the shaft,
you did something wrong. Start over.

One fix you don't want to have to do is locktite an oversized pin into the
end of the shaft and then machine it while you traverse the axis. Don't
use a hardened drill blank for this approach. I've installed lots of
encoders and this is a distillation of those experiences.

Ted Robbins