By John Pieri, Sr. Product Line Manager,
Thomson Deltran Clutch Brake
Medical imaging equipment, wa- ter handling systems, convey- ors, robotic systems and rotary
and linear actuators are among the many
devices that may be fitted with electric
friction brakes to hold their loads in place
when the power is off or disrupted.
Because the inclusion of a brake has
significant implications for the entire
design, especially for determination of
size and the selection of power supply,
they must be designed in from the start.
One mammography system designer
learned this the hard way, not realizing
the need for a braking system until he
was testing the prototype. Fortunately,
engineering support and an express
fulfillment program from Deltran brake
manufacturer Thomson Industries, Inc.,
enabled him to rescue a failing design
and meet the schedule for the original
design and prototype.
Why mammography systems need
Mammography devices typically use a
C-arm shaped apparatus in which an
X-ray tube projects downward from
the top of the C to scan the body
generating a precise blur-free image
that could reveal indications of breast
cancer. A rotating ball screw bracketed
to the tube assembly turns slowly -
usually only a few hundred revolutions
per minute - move the scanner evenly
across the target area.
Because the carriage must change
direction many times in any session,
any play resulting from gaps between
components affects positioning precision
and image quality. This loss of motion,
commonly called backlash or backdrive,
is also a potential problem when the system is at rest, when it can cause noise,
vibration, and wear.
Electric brakes help control the backlash while the system is at rest and help
bring things to a smooth stop if there is
sudden loss of power from motor failure,
power outage, or other event (Figure 1).
Loss of electric power de-energizes
the brake linings to grip a rotating plate
(Figure 2), stopping it from turning
or holding it in place once it is at rest.
Re-energizing the system disengages
the brake, allowing the shaft to rotate
freely once again. However, the designer
mentioned above did not discover the
need for such a braking system until it
was almost too late.
Not considering the need for a braking
system, the original motion control com-
ponent design called for the following
• Backlash less than 0.5 degrees
• 2 Nm of holding torque
Figure 1: Mammography systems rely on electric brakes to control unwanted movement
of their patient positioning mechanism.
A real-world case study gives practical tips for controlling backlash
in your next design.