Repairing the backgear on my Atlas Lathe
When I purchased my lathe in May of 2011, I was warned that
there was a problem with the backgear. I was also encouraged
by predictions that it probably would not be hard to set right.
Here is a picture of my headstock showing the backgear and
almost all of the relevant moving parts:
Some parts have been removed for cleaning and repair, but
everything relevant to the backgear is in clear view here.
The backgear is the pair of gears on the shaft to the rear.
This assembly can be moved in and out of engagement by the
use of a lever (not visible) on the right.
When engaged, the backgear reduces all speeds by a factor
of approximately 6.0.
How it is supposed to work
There are 3 parts on the spindle that get involved with the backgear:
- The small gear on the left (which is engaged by the big gear
on the backgear assembly).
- The pulley cluster.
- The "bull" gear on the right (which is engaged by the
small gear on the backgear assembly).
The bull gear is rigidly attached to (and drives) the spindle.
When the backgear is not engaged, a belt drives the pulley cluster,
which is rigidly coupled to both the bull gear and the small gear.
The small gear is just along for the ride.
The bull gear can be disengaged from the pulley cluster.
This is done by pulling a pin to the right.
After this is done, the backgear can be pivoted into engagement.
Engaging the backgear without retracting the pin will lock up
the spindle, and probably break things if power is applied.
With the backgear properly engaged, the belt drives the pulley cluster and small gear,
which are not coupled to the spindle, they just revolve freely.
The small gear drives the big backgear, and the
little backgear drives the bull gear, which is fixed to the spindle
and away we go.
How mine is screwed up
There are two problems, both of which, in my analysis trace to an error
made when parts were reassembled on the spindle.
- First, when the pin in the bullgear is pulled to the right, it does not
go far enough to release the pulley cluster. The two can never be
uncoupled, making it impossible to use the backgear.
- Second, the small gear slides left and right on its shaft.
When it is to the right, it has "teeth" that engage the pulley
cluster (as it should always do). When it is to the left, it comes
out of engagement, but is always in danger of migrating right and
reengaging.
My analysis is simply that the pulley cluster is mounted too far to the
right. If it was moved left it would permanently engage the small gear
(trapping it against a collar), and it would be far enough away from the
bull gear that the pin, when retracted would not mate with it.
More information
Have any comments? Questions?
Drop me a line!
Tom's home page / tom@mmto.org