Article: Courtesy of Ruland
Manufacturing Co., Ltd.
Shaft Collars are one of the simplest components
in power transmission and are the most indispensable.
They can be found in virtually any type of machinery,
and are frequently accessories to other components.
Among their many roles, shaft collars hold bearings and
sprockets on shafts, situate components in motor and
gearbox assemblies, and serve as mechanical stops.
For more than 70 years, Ruland Manufacturing Co., Inc.
has been producing “thoughtfully designed” and
“carefully made” products from the Marlborough, MA
manufacturing facilities. Through continuous innovation,
manufacturing control and a wide-range of proprietary
processes including special materials, surface
treatments, anti-vibration coupling hardware and
precision honing, Ruland continues to drive the
evolution of product performance and appearance.
The first mass produced shaft collars were solid ring
collars which utilized square headed set screws which
protruded from the collar. These early collars were
considered dangerous because of the workers risk of
catching their clothing on the protruding screw head.
Set screw collars have come a long way since those early
days and are now manufactured with a recessed set screw.
Set screw collars derive all of their holding power from
the screw as it is tightened onto the shaft. The amount
of holding power depends greatly on the material of the
shaft on which the collar is installed. In order for a
set screw collar to obtain maximum holding power, the
shaft must be of softer material than that of the set
screw. This allows impingement of the screw point into
the shaft keeping the screw and collar in its installed
position under torque and axial loads, instead of
sliding along the shaft.
The very nature of the set
screw collar is its greatest fault. The tightening of
the set screw causes marring on the shaft which is
undesirable for functional as well as cosmetic reasons.
The impingement of the screw causes an eruption of
material around the point resulting in a raised burr on
the surface of the shaft. The marring of the shaft
makes it difficult to remove the collar or fine-tune its
position. Small angular or lateral adjustments are
almost impossible to make, since the screw point will be
drawn back to its original location.
The first mass-produced collars were
used primarily on line shafting in early manufacturing
mills. These early shaft collars were solid ring types,
employing square-head set screws that protruded from the
collar. Protruding screws proved to be a problem because
they could catch on a workmen’s clothing while rotating
on a shaft, and pull them into the machinery.
Shaft collars saw few improvements until the early 1900’s when
Howard T. Hallowell created the first recessed head
socket set screw shaft collar. Hallowell received a
patent on his safety set collar, which was soon copied
by others and became an industry standard. The invention
of the safety set collar was the beginning of the
recessed-socket screw industry.
Set screw collars derive
all their holding power from the screw as it is
tightened onto the shaft. Holding power depends greatly
on the shaft material and condition. For a set screw
shaft collar to achieve maximum grip, the shaft must be
of softer material than that of the set screw. This
allows for impingement of the screw point into the
shaft, which keeps the collar in its installed position
under torque and axial loads, and prevents it from
sliding along the shaft.
Set screws tend to damage
shafts which is undesirable for functional and cosmetic
reasons. Screw impingement causes an eruption of
material around the screw resulting in a raised burr on
the shaft surface. This raised material makes it
difficult to remove or reposition the collar. The
invention of the clamp-style shaft collar solved the
destructive issues associated with the set screw. Clamp
collars utilize compressive forces to lock onto the
shaft without any damage.
Nobody is certain who first
invented the clamp-style shaft collar, but they have
been around since WWII when they were used in bombsights
and guiding systems. These mechanical instruments
consisted of precision gearing, differentials,
couplings, and collars in combination with electrical
selsyn motors, resolvers, precision potentiometers, and
a variety of electronics. At the time, they were
considered high tech, top secret, and were cutting edge
An improvement from the set screw collar, the clamp
style shaft collar does not mar the shaft. While it is
not known with certainty who invented the clamp style
collar, Ruland Manufacturing was developing them for
bomb sights and guidance instruments during World War
II. These seemingly basic mechanical instruments were
considered very high tech (and top secret) at the time
and were the forerunner of the analog computer
industry. While they are taken for granted today; at
the time, clamp style collars were the cutting edge
precision components required for such advanced
Clamp style collars solve many of the
problems that exist with the set screw collar and are
available in one and two-piece designs. Instead of
marring the shaft with a set screw, clamp style collars
utilize compressive forces to lock the collar onto the
shaft. For this reason, clamp style collars are easily
removed, indefinitely adjustable, and work well on
virtually any shaft. Tightening a few clamp screws
closes the collar onto the shaft for a nearly uniform
distribution of forces around the shaft’s
circumference. The uniform clamping is mechanically
more secure than point contact, as much as doubling
holding power, depending on shaft size and condition.
Although clamp type collars work very well under
relatively constant loads, shock loads can cause the
collar to shift its position on the shaft. This is due
to the very high forces that can be created by a
relatively small mass during impact, compared to a
statically or gradually applied load. As an option for
applications with this type of loading, an undercut can
be made on the shaft and a two-piece clamp collar used
to create a positive stop that is more resistant to
This approach demonstrates the benefits of the two-piece
design. It has more holding power than a one-piece
design because it uses its full seating torque to apply
clamping forces to the shaft. One-piece collars
sacrifice some of their clamping ability because the
screw must use a portion of its seating torque to bend
the collar around the shaft.
Two-piece collars also boast advantages in installation
and assembly. While set-screw collars and one-piece
clamp collars must slide over the end of a shaft,
two-piece collars can be disassembled and installed in
position without having to remove other components from
the shaft. In the case of an undercut shaft, a one
piece collar has to be pried open to accommodate the
shaft’s diameter while a set-screw collar cannot be
installed properly in any case.
Another approach to increase secure positioning under
shock loads is to stack several collars, or to add pads
or bumpers. Multiple collars increase load capability
due to the additional clamping force provided by
multiple screws and the frictional benefits of increased
shaft contact. Double or other extra-wide clamping
collars with multiple screws have the same advantages as
a stack of collars. Bumpers or pads absorb some of the
shock load and can also reduce some of the noise caused
Clamp-style shaft collars perform critical
duties. Choosing the best shaft collar is a matter of
matching one or more performance factors with specific
application requirements. In many applications, the
collars holding power is paramount. Other important
factors include its ability to weld, inertia,
conductivity, corrosion resistance, and collar-face
precision as it relates to the bore.
Holding power is
the most important feature when the collars are used on
split hubs or as mechanical stops. (Split hubs are
interfaces for connecting components such as gearboxes,
sprockets, encoders, and couplings to shafts.) Collars
are effective split-hub clamps, but the application is
particularly demanding since a portion of the clamping
force is expended closing the hub, which reduces the
forces applied to the shaft. Maintaining close
tolerances between the shaft, hub, and collar (and
keeping hub thickness as small as possible helps
minimize the amount of force lost on the hub itself.
Several design and manufacturing features such as bore
size and concentricity, influence the holding power of
clamp-style shaft collars. However, many of the most
important factors are related to the fundamental
mechanics; a function of the amount of screw torque
indirectly transmitted to the shaft by frictional forces
between the shaft and bore.
Screw size and quality also
factor into holding power. Some of the many attributes
of superior screws are thread quality, tensile strength
of material, and closely held geometry and size
tolerances, which eliminates frictional drag on the
collar socket. In general, forged screws are superior
to those that are broached. But with so many attributes
to consider, comparisons are best made empirically.
Material strength and collar design are factors
affecting the translation of screw torque in to shaft
collar holding power. The material needs to be strong
enough to withstand the recommended screw torque.
Low-grade materials could crack or deform under torque,
reducing holding power and possibly result in
catastrophic failure. The threads, the bottom of the
counter-bore, or the sides of the counter-bore, can all
deform, resulting in reduced collar performance.
It is a common misconception that a larger outer
diameter makes a collar stronger. A larger outer
diameter has the advantage of being able to hide the
clamp screws within the shaft collar, rather than be
left protruding. But unless width is also increased
allowing for a larger screw (which would be protruding)
to be used, increasing shaft collar outer diameter
relative to the bore size has no performance benefit and
can reduce holding power. Holding power can be reduced
for the reason that some of the screw torque must be
expended to bend the collar around the shaft before the
remaining forces are applied to the shaft.
To create extra clamping force, a larger screw must be
used or the screw location must be moved away from the
center line of the shaft to create greater mechanical
advantage, resulting in increased holding power.
Increasing the outer-diameter of the collar without
changing the screw size or location only creates a
condition where excess material must be deformed
elastically before any forces are applied to the shaft.
Some forces will be expended this way regardless but
these can be reduced and holding power increased by
having the collar OD no larger than necessary.
Holding power can be increased further by having a
back-cut opposite the clamp-cut in the bore of a
one-piece clamp collar, thus reducing the material at
the collar’s hinge point. Using a two-piece collar
further reduces the amount of material to bend and has
the added advantage of a second screw to transmit torque
but the result is only an approximately 2% increase in
holding power over a properly designed one-piece clamp
collar. This small increase may not be worth the added
cost of the two-piece design, especially if it adds
complication in design and installation such as the need
access to both screws when the collar is in an
Although the holding power gains may not be significant
and the one-piece design is often more convenient,
two-piece collars have the advantage of easy disassembly
and removal from the shaft without needing to remove
other components. Two-piece collars can also be
balanced more easily (by opposing the screws) than
one-piece collars. Balancing is sometimes necessary
where the collar rotates at high RPM, as may be the case
with split-hub applications.
Tightening a screw is a deceptively simple task. When
it’s done to attach collars to shafts, any number of
difficulties can arise. One common problem is
stick-slip. Stick-slip can create a false impression
that a screw has been tightened to its appropriate
During tightening, a screw rotates uniformly as it’s
torqued down, then it reaches a point where its rotation
gets sticky. The screw begins to turn in a choppy
manner, stopping and starting even though tightening
torque is constantly applied. The lost torsioning
effort during stick-slip is typically absorbed as excess
friction between the threads or underside of the head
and the mating parts of the clamp body, instead of
contributing to the stress in the joint elements.
If these stresses in the joint are too low, the collar
will not hold well. The best way to avoid the effects of
stick-slip is by using specially coated elements.
For example, black oxide helps to smooth the torqueing
of screws without diminishing the frictional
characteristics and holding power of the bore.
Clamping screws that operate smoothly during torqueing
are the best assurance that stick-slip is not present.
Another important contributor to shaft collar holding power
is the surface treatment of the collar and screws. The most
common shaft collars are steel with black oxide finish,
which enhances the torque of the screws yet does not
significantly diminish the frictional characteristics of the
bore with a net increase in holding power.
This increase can be optimized through combination of the
black oxide formulation and a well-chosen light oil
treatment on the screw. Alternative surface treatments such
as zinc have better corrosion resistance than black oxide
but tend to significantly reduce holding power
Black oxide is effective partly because it is an anti-stick
slip compound. Stick-slip is the false impression that a
screw has been tightened to the appropriate stress level.
Instead of the screw rotating uniformly as the torqueing
continues, there is a point at which the uniform rotation
converts to a stop and start pattern. The tensioning effort
on the screw is being absorbed as excess friction between
the threads or the underside of the head and the mating
parts of the clamp body, instead of contributing to the
stress in the joint elements. If the stresses are low, the
collar will not hold well.
The stick-slip condition can be elusive. A silky smooth
operation of the clamping screw during torqueing is the best
assurance that stick slip is not present. Properly
formulated black oxide helps to create this condition.
Stick-slip compounds and other surface treatments on the
screw threads such as zinc, molybdenum, or nylon alter the
tightening characteristics of the screw. If the normal
torque is applied to the screw the most likely result would
be an over-torqueing of the system with the risk that the
screw or the collar could be damaged and subject to sudden
failure. Furthermore, the unevenness in application of
some screw thread surface treatments may make the torque
inconsistent and lead its holding power to be unpredictable.
As well as their holding power, a primary performance
characteristic of shaft collars is their ability to
locate and align other shaft components with a precisely
machined bearing face. In order to assure this level of
precision, high performance shaft collars are single
point faced at the same time that the bore is finished.
This results in very low run-out when the collar is
mounted on the shaft, which is extremely important when
interfacing with other precision components.
Clamp-style shaft collars in both one- and two-piece
styles allow for easy installation and adjustment and
are used commonly to locate components on a shaft, such
as sprockets, gears, pulleys, and ball bearing units.
In these cases the ability to retain axial loads is
important, but the perpendicularity of the collar face
to the shaft is critical.
A precise face-to-bore relationship ensures squareness
of the component to the shaft with no shifting or
tilting relative to the shaft axis. Such displacements
can cause premature wear and possibly affect the
performance of the assembly. In cases where sprockets
or pulleys are used with chain or belts, the alignment
of the component is critical to proper operation.
Failure to maintain alignment can result in unacceptable
performance, including excessive noise, slippage,
whipping, rapid wear or total failure, depending on the
A perpendicular face also ensures even pressures at the
interface with the mounted component, eliminating spot
loading which can shorten life of the components. In
applications where collars are used against bearings,
this is an extremely important characteristic since
uneven loading of bearings is detrimental to long life
and high performance.
This is also beneficial in applications that see
moderate axial shock loading such as linear actuators.
In applications such as these, where the collar is used
as a mechanical stop, face squareness is important to
assure even force distribution across the face of the
collar to minimize the impact pressure and ensure that
the collar does not shift position on the shaft.
A popular shaft collar variation is the threaded bore
collar. Typically, threaded collars are available in the
same styles as smooth bore collars, but are most popular in
the one and two piece clamp styles. This is because set
screw styles would do large and permanent damage to the
threaded shaft the collar is installed on, by impinging the
screw into the threads. Clamp-Style threaded collars open
possibilities for many applications using threaded shafts,
but stand out particularly in two areas; application with
high axial loads, and applications requiring fine location
or preload adjustments.
For high axial-load applications threaded collars have a
distinct advantage over smooth-bore collars which rely on
friction for resistance to axial loads, making them
susceptible to movement when shocked. Threaded collars have
a positive mechanical stop created by the interface of the
threads on the collar and the shaft, making the collar
almost impossible to move axially without breaking the shaft
Clamp-style threaded collars also make it easier to perform
fine adjustments and / or preloading of components such as
bearings. This is performed by simply threading the collar
into location and locking it in place by tightening the
screw to proper torque levels.
Bearing locknuts are a special designed threaded collar
intended solely to mate up with bearings. They have a more
precise control tolerance for face run-out to the threads to
ensure even pressure on the entire bearing face and precise
control of preload. Typically, spanner wrench slots are
machined into the outer diameter of the collar to allow for
easy access and precise adjustment of this preload. As with
other threaded collars, once the preload is established, the
locknut can be secured by tightening its screw.
Quick-clamping shaft collars are relatively new to the shaft
collar market. Much like the clamping type collars, they do
not mar the shaft. The greatest advantage of the
quick-clamping shaft collars is they do not require tools to
install or remove them, and are especially designed for
quick adjustments. They also work well with light duty split
hub components such as gears and sprockets and they can be
machined to facilitate mounting of other components.
Quick-clamping shaft collars are ideal for light duty
applications with frequent setup changes or adjustments.
The quick clamping collars are designed with a low profile
clamping lever which makes them suitable for rotating and
stationary applications. Industries such as printing and
packaging, as well as others, can benefit from the use of
quick clamping collars to retain frequently changed items,
such as rolls of media, or for rapid and precise adjustment
of guide rails or other setup alterations.
Shaft collars are versatile components that have evolved
from the basic set-screw design to high performing versatile
clamp styles. Shaft collars are produced in a variety of
materials and in a wide range of sizes, making them
convenient solutions to a variety of engineering problems.
Many shaft collar applications are demanding, so selecting
shaft collars with the correct features required for top
performance is vital to equipment functionality.
© Ruland Manufacturing Co., Inc. All rights reserved.