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Application of the Month
Reciprocating Motion Without Servos
March, 2007
Shaft phasing gearboxes can be used for more than just registration
and timing applications. This customer found a unique solution
by incorporating this dual input gear drive design.
Shaft phasing gearboxes go
by a variety of names such as speed correction drives and timing
differentials. The differential moniker came from the original
bevel gear designs, with side spider gears, that could be rotated
with or against the direction of the main drive gears. This had
the effect of speeding up or slowing down the gearbox output
speed relative to the fixed input speed. This resulted in the
ability to accelerate or decelerate a driven element to move,
or phase, it to a new position.
Newer designs use planetary
gear designs, with moveable rings, to achieve the same result.
This provides a more compact drive and is easier to incorporate
with bevel gearing for a right angle solution. A typical application
would be a printing press, or other multi stage process, that
requires precise motion and timing from station to station, but
is driven from a single power source. With a fixed mechanical
connection, adjustments at each station are usually difficult.
With a shaft-phasing gearbox a secondary input allows output
adjustments independent of the rest of the linked system.
A customer came to us with
an application that allowed use of this technology in an entire
new way. He was designing a relatively simple packaging machine
that required synchronized movements of relatively heavy packages.
Today many of these types of movements are achieved by servo
motor control. However, this customer wanted a more reliable
mechanical system. In addition, the torque required to execute
the simple motion on the heavy load would have required a large
independent servomotor. So he was looking for a different way.
The profile was not complex.
A product came into position and a push rod mechanism moved it
to the next position. The push rod then retracted to await the
next product in the cycle. This continued in a synchronized manner
with the other machine work stations. The obvious issue here
was that it took a lot of torque to move the load but almost
none to retract the push rod mechanism. And the question was
how to do this without an expensive, independent servo.
A key to the solution was that
the movement of the load was relatively slow. In a typical phasing
gearbox the correction shaft usual has a very high ratio for
extremely precise phase adjustments. But in our case, we offer
a variety of ratios so that larger adjustments can be achieved
more quickly. It's this optional ability to make quicker adjustments
that fit the bill.
Our concept was to take power
off the main machine prime mover. There was excess capacity and
this provided the synchronize speed with the rest of the machine
operations. By connecting the output of a phase box to the push
rod mechanism, we had all the torque we needed to push the load.
But how to retract the rod at the end of the stroke? This was
achieved by adding a low power, high-speed motor to the correction
shaft.
When the push rod reached the
end of its stroke, it activated a limit switch, which in turn
started the correction motor. This retarded the output speed
of the phase box to zero and then to reverse rotation to withdraw
the push rod back into position. When it reached the home position
another limit switch turned off the correct motor, which stopped
the phase adjustment, allowing the original gearbox speed and
direction to resume, which drove the push rod to move the next
package that entered the station.
In this design the input speed
and direction of the phase box was continuous, in synch with
the rest of the machine. But by using simple limits switches
and an inexpensive drive motor, the output shaft could independently
be stopped and driven in the reverse direction to achieve the
desired result.
While this isn't a typical
application, it is typical of the drive challenges DieQua is
presented with every day. While we provide a lot of gearboxes
and speed reducers for a lot of standard applications, we specialize
in finding new and unique ways of using these products to satisfy
specific customer requirements. If you have a particular problem
you are trying to solve, there's a good chance we have a gear
product or design concept to help you solve it.
Chris Popp
Director of Marketing
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