Balancing an Elephant on a Beach Ball
Selecting the right drive train for motion in your machine is kind
of like balancing an elephant on a beach ball. Multitudes of key
operating characteristics are both required and desired for machine
design. Each of the different potential drive trains that can be
applied to solve your motion problem has a unique performance
in each of the characteristics relative to the other drivetrain technologies. Therefore, making the right selection is not always about
maximizing a single characteristic — instead it is about defining an
optimal level on a number of different fronts. In other words, a good
selection results from reaching a point of equilibrium by balancing
the most important of the key performance characteristics.
The list of potential performance characteristics that might be
of interest is significant. To focus on the selection process, all the
options are classified into four major categories, including precision,
expected life, throughput, and special considerations (PETS).
The Five Common Drive Trains
The five most commonly used drive trains in linear motion are ball
screw, lead screw, timing belt, rack & pinion, and linear motor. When
other technologies or form factors might work for an application, the
basic constructs and prioritization methods should still apply.
Ball screw drive trains include a threaded rod and matched ball nut
with recirculating ball bearings between the nut and screw interface
surfaces. The rolling ball bearings provide for high efficiency and
Ball screws are ideal for applications requiring high precision,
repeatability, and force density. Ball screws can achieve moderate
speeds, but are limited based upon screw whip. It can be relatively
difficult to properly align the screw to the line of travel to ensure
maximum life and minimize the maintenance required. In addition,
a ball screw can be quite noisy depending upon the grade of the
Lead screw drive trains include a threaded rod and matching
threaded nut sliding interface. In some cases, the nut is preloaded
against the screw to reduce backlash.
Lead screws work well for low duty cycle applications, or applications requiring small adjustments. Lead screws are typically about
half as efficient as a ball screw, so they require about twice the
torque to achieve the same thrust output of the screw. One advantage of the technology’s low efficiency is its inherent resistance to
back driving, which can be ideal for vertical applications that would
otherwise require a brake to hold the payload under loss of power.
Timing belt drive trains include two cogged pulleys, typically one
driven and one idler, tied together with a timing belt that has a
carriage attached. This technology is the most simple and common
drive train in linear motion.
Timing belts are a robust mechanism for high-speed applications
requiring long life and maintenance-free operation where precision
greater than 100 microns is required. Design hurdles associated
with belt drives include the need for a reliable belt tensioning system as well as rugged pulley bearings to handle thrust loads.
Rack & Pinion drive trains include a machined linear gear (rack)
and round mating toothed gear where the round gear is typically
mobile and the rack stationary. Rack and pinions are suited for very
long travels requiring high speed; however, they are not a precision technology because there is no good way of removing system
backlash from the drive train. In addition, rack and pinion drives are
often quite noisy.
Linear motors include a row of magnets that interfaces with an
electromagnetic carriage to move a load in a linear direction. Linear
motors are direct drives without the mechanical advantages and
disadvantages of the other common drive trains.
Whether you are designing your own motion solution, selecting components from
a supplier, or working with a system
solutions provider, understanding drive
train selection is a key to a successful
and reliable machine. In this endeavor,
it’s important to understand the
performance characteristics of each
drive train technology.
The NUTS & BOLTS
of Selecting the Correct
Linear Drive Train
Take the guesswork out of choosing the right linear
drive train for your next positioning application.
By Jim Monnich, Engineering Manager, Parker Hannifin