MDTmag.com 22 / September/October 2017
time, the brake tang engages with the braking stop, causing the
brake spring to wrap down and stop the output hub.
Medical Apps Benefit
Wrap-spring clutches and brakes offer
a number of advantages for medical
applications compared to friction-based
designs, starting with performance.
Wrap-spring designs provide high-torque
capacity in a compact package. This
is particularly important in space-con-strained applications like surgical
robotics and medical pumps. A size six
wrap-spring clutch is about 2 inches in
diameter and can deliver 500 lb-in of
torque. A friction-based version would have to be 5 inches in
diameter in order to achieve the same torque rating.
Wrap-spring devices operate with a positioning accuracy of
±5º. They offer response times of roughly 3 to 5 ms, compared
to 40 to 120 ms for friction-based designs. As a result, they
are good fits for applications with precise timing and positioning requirements. Fast response times can be a significant benefit, but also require informed design. In the case
of a safety brake on a wheelchair lift or a surgical robot, for
example, fast response may be highly beneficial. It is probably
less appropriate for a patient-positioning application like an
Medical equipment like automated pharmaceutical dis-
pensing systems are designed for high use and long lifetimes.
Wrap-spring devices are made of oil-impregnated, sintered
steel. As such, they are considered lubricated for life. With
the exception of applications with very light loads, wrap-spring clutches and brakes do not need internal bearings. This
eliminates both maintenance and points of failure.
Heat management is an important attribute in patient-facing equipment. Specifications often put temperature limitations on support equipment, for the protection of both
maintenance technicians and enterprising medical staff alike.
Wrap-spring devices generate minimal heat. The only source
of electrical heat is the solenoid used to activate the control
tang, but that is a small, infrequent movement that generates
The points above bring up another notable benefit of
wrap-spring clutches and brakes they consume minimal power, which can be important for portable applications.
The devices also speed manufacturing. Wrap-spring clutches and brakes arrive as self-contained units. For simple designs,
the units just slip onto a shaft. When they need to be coupled
to a pulley or a second shaft, they can be connected using
flexible or loose couplings.
When it comes to custom devices, wrap-spring clutches
and brakes truly shine. Many engineers with designs requiring
large amounts of torque in a small diameter choose toothed
clutches and brakes. The problem is that these devices are
either machined or made out of molded, patterned medical
metal. Depending on performance levels, cost of tooling can
run quite high. In contrast, wrap-spring clutches and brakes
require minimal tooling. As a result,
they lend themselves to customiza-
tion and can be as much as five times
less expensive than a comparable
As with all things in engineering,
wrap-spring brakes and clutches
involve trade-offs. The basic sin-
gle-spring wrap design is unidirec-
tional. Bidirectional operation requires two separate devices.
Alternatively, bidirectional devices with multiple springs have
been developed (see sidebar).
The actual operating speed range for wrap-spring clutches
is lower than for friction designs. Inherent drag between the
spring and the hubs limits speeds. Wrap-spring devices should
not be used above 1750 RPM. This generally does not present a
problem for medical applications since patient-facing equipment
tends to operate more slowly.
Medical device OEMs might face conflicting demands, but
wrap-spring technology provides a solution. Wrap-spring devices
deliver high torque in small packages, high performance from
simple devices. They are easy to install and cost effective. Most
of all, they are robust and reliable, delivering failsafe performance
for even the most demanding patient-facing application. For engineering teams looking to deliver better products to market faster,
a wrap-spring clutch or brake might just be the secret weapon
that gives them an advantage over the competition. MDT
Figure 4: Adding a control tang and a stationary braking hub to
the output side converts the basic design brake. (Image Credit:
With the exception of
applications with very
light loads, wrap-spring
clutches and brakes do not
need internal bearings.