|
Application of the Month
Driving Screws
July, 2007
Linear motion for the positioning
of tooling, or parts to be worked on, is a common requirement
in automated machinery. Where dual drives are required, precision
gearing is critical for positioning performance.
In many applications a single
linear device, such as a ball screw or belt drive, can be driven
by a single servo motor, which may or may not include a gear
head to gain additional mechanical advantage. But, for larger
or wider loads dual screws are necessary to provide even or balanced
forces. While synchronized servo motors could be used to independently
drive each ball screw, precision gearing allows the use of a
single servo motor, reducing the expense and complexity of electronic
control.
A customer of ours recently
requested a drive line solution to mechanically link two ball
screws, to allow accurate positioning, with a single servo motor.
Information on several different elements was necessary to offer
the best solution. These included torque, speed, and available
mounting space. Several options were available. Determining the
best combination required consideration of all these elements.
The typical solution is to
drive the ball screws with precision right angle spiral bevel
gearboxes that are connected by a torsionally rigid line shaft.
There are several ways this can be done, each with its pluses
and minuses, depending on the parameters.
Perhaps the best mechanical
solution is to have a center driven right
angle box going left and right to each corner gearbox driving
each ball screw. This has several advantages. It provides a relatively
compact solution, keeping the drive inside the width envelope
of the machine. It provides consistent input motion to both corner
gearboxes. It also provides the possibility of two stages of
speed reduction to optimize gearbox and line shaft sizes, as
well as inertia matching for the servomotor.
A point of consideration in
this and other applications like it is the span and desired speed
of the driving ball screws. In this case the overall motor speed
to screw input speed was to have a 2:1 ratio. The question was
where best to put the 2:1 reduction, in the first drive box or
the corner boxes?
To keep all three boxes the
same size, it is best to put the ratio in the corner boxes. These
boxes would see half the output torque of the main 1:1 drive
box. Doubling the torque through the corner box would mean each
box would generate the same output torque, regardless of its
location.
But in this instance, the line shafts would be
operating at motor speed. If the distance between the boxes was
large and the lineshafts long there could be a possibility of
shaft whip or the potential of unwanted frequencies. Calculations
would have to be performed. On shorter lengths the advantage
is in using a smaller and less costly shaft because of the lower
torque being carried.
If the initial drive box gets
the ratio, there are other advantages and disadvantages. The
down side is the first box sees all the torque, so it will get
bigger and more expensive. With the multiplied output torque,
the line shafts also have to get larger. The only real advantages
are that the line shafts run slower and there may be some reflected
inertia improvements.
This configuration can work
even better if the required speed reduction ratio is higher.
The possibility of having ratios in two boxes provides a lot
more design versatility.
In many of these applications
there is no mounting structure between the screws to install
a center drive box. In these cases we have fabricated a mounting
flange to what is typically the output side of a "T"
style 1:1 bevel box. To this "input" flange we have
mounted the servo motor directly, for ratios of 2:1 or less,
or either an inline
planetary gear head or right angle servo
worm when ratios of 3:1 to 5:1, or higher, are required.
Half the torque is taken by the gear set of the first screw drive
box with the remainder passing through to the second screw drive
box, via a line shaft. Because of the longer span between boxes,
the line shaft considerations previous discussed will apply.
While some will be concerned
about lost motion at the second screw, we accommodate this by
using torsionally rigid torque
tubes with flexible ends to compensate for shaft misalignment.
In a screw drive any potential lost motion due to shaft wind
up is minimized by the mechanical advantage of the screw ratio.
This is rarely an issue except in the longest of spans. And larger
diameter torque tubes can always be used to increase rigidity.
When space is really at a premium
we suggest our Servofoxx
gearhead which has an optional auxiliary
drive shaft. In this design we can mount the servo motor,
get a speed reduction into the first screw and have an auxiliary
shaft extension to connect the line shaft, all in one unit. It
is compact and reduces the number of components to be mounted.
It is normally customized for the application, due to the wide
variety of ratios and configurations available, therefore it
is not an "off the shelf" offering. However, it is
a unique problem solver not available elsewhere.
Using mechanically linked drives
for synchronized motion is really the most reliable design, especially
when both axis are following the exact same motion profile. While
servo motor manufacturers will lobby for independent drives,
the increased complexity of the control and feedback functions
will add unnecessary performance risks. Typical anti-mechanics
arguments made by electronic proponents may apply when using
standard industrial gear drives. However, DieQua supplies gear
products and connecting components designed and optimized for
the speed and positioning requirements of the most demanding
automated equipment being developed today.
For these and other types of
automation applications were the "mech" part of mechatronics
is critical, give DieQua a call. We will help you choose the
right product for your unique needs and give you advise on how
best to integrate it into your motion system design.
Chris Popp
Director of Marketing
--------------------
Did you know that DieQua publishes
a monthly email newsletter that features interesting articles
just like this one?
Our newsletter was designed
to be a little different than other manufacturer's newsletters
that are primarily product centric. While products and services
available from DieQua are highlighted, other elements revolve
around the personal and professional development of the design
engineer.
To receive this newsletter,
please visit the registration
page and fill out your information.
|
