Brakes Control in Motion
Patient Rehab Equipment
By Rocco Dragone, Senior Applications Engineer, SEPAC
Medical-device OEMs face conflicting demands at every turn. The safety of patients and personnel is paramount, but
performance is a close second. Moving equipment needs to start
and stop on demand, and it needs to do so accurately, rapidly,
and reliably. Brake and clutch assemblies need high enough
torque capacity to control the load, but they need to
be compact. They should offer
positive engagement to
ensure safety, but they
need to be simple. For
portable devices, power consumption needs
to be minimal, and for
any device, cost control is
always a concern.
have become a standard
solution in medical devices, but
they have drawbacks that include
complexity, delayed response time,
higher power consumption, and higher
cost. What if there was a better alternative?
What if there was a clutch/brake technology that combined high
torque capacity, millisecond response time, ease of use, and small
form factors? And what if it was a simple mechanical solution with
a long track record of safety and reliability in the industrial market?
That solution exists. It is the wrap-spring clutch/brake.
A wrap-spring device is formed of three elements: an input hub
(motor side), an output hub (load side), and a helical spring
that wraps around both hubs to connect them (see Figure 1).
The inner diameter (ID) of the spring is smaller than the outer
diameter (OD) of the hubs. When the input hub turns, the spring
tightens (wraps down)
around both hubs, effec-
tively locking the two
together by natural force.
When the input hub stops or
reverses, the spring unwinds to release the output
hub. This basic mechanism can be modified to operate as
a variety of different clutches and brakes.
The basic wrap-spring mechanism as described above forms
an overrunning clutch. The clutch can be used to accelerate a
load from a stop by using the input hub to drive the output hub
via the spring. When the input hub stops or reverses, the spring
releases to enable the output hub and load to coast (overrun).
The basic overrunning clutch design can be modified for a
number of specialty functions. This requires the addition of a
control tang at one or both ends of the spring. The location of
the tangs and how they are applied determine the function of the
The basic wrap-spring clutch design can be converted to a
start-to-coast clutch for random positioning by adding a stop collar and a control tang on the input-hub side of the assembly (see
Figure 2). In this mode, the input hub starts the load the load
via the standard mechanism. The load is released when the tang
on the input side of the spring engages with a stop machined on
the inside of the stop collar, causing the spring to unwrap and
disengage from the output hub.
Figure 1: A wrap-spring clutch/brake uses
a spring to connect and input of (motor
side) to an output hub (load side).
When the input hub turns, the spring
tightens around the output hub
(wraps down) to apply torque
and accelerate the load. Stop-
ping the input hub unwinds
the spring to release the
load. (Image Credit: SEPAC)