FLEXIBLE SHAFT
COUPLING PRIMER
INDEX
SUBJECT
PAGE
General Information
1
Bellows Type Couplings
3
Metal Bellows Corporation
Servometer Corporation
Disc Type Couplings
4
Thomas Rexnord
Renbrandt
Multi-overlapping Curved Beam Couplings
5
Rocom Corporation
How To Select Couplings
6
Misalignment
7
Misalignment Cause and Effect
7
Common Terminology
8
GENERAL INFORMATION
The flexible coupling market is very horizontal with a number of strong vertical
segments, such as:
Medical equipment, including respirators, blood analyzers, hospital beds and a
number of lab machines;
Small motor pumps;
Computer peripherals such as line printers and tape drives;
Motor driven actuators;
Military radar and communication equipment.
Probably the largest vertical segment, and certainly the hottest, is the CNC
machinery field including robots, pick-and-place equipment and computer-controlled
inspection equipment.
Each automatic machine manufacturer wants to claim the shortest length of time
and the highest accuracy to move from point to point to perform its operations. The most
accurate encoder is of no value unless the couplings accurately transmit the information
they are given. To perform its operation there are three critical factors, and these are;
(1)
WINDUP - has always been recognized as a problem. If there is a great deal of
windup, the ramp down time starts too soon or too late; but windup is not nearly
as significant as hysteresis or lost motion.
(2)
HYSTERESIS - The most critical thing is that the coupling tells the encoder
where the shaft is when it stops. Arc minutes of lost motion can be interpreted
into thousandths of an inch where the machinery manufacturer is trying to hold
ten-thousandths of an inch.
(3)
ROTARY OSCILLATION TIME - is the length of time it takes for a flexible
coupling to stop its rotary oscillation or resonating frequency when the system
comes to a stop. The rotary oscillation determines the length of time the machine
must stop to find out where it is before performing its functions.
-1-
Rocom couplings have been tested by General Electric-Charlotesville, Electrocraft, BEI
Encoder Company, Dynamics Research Encoders, Westing-house, Otis Elevator, Moog
Corporation and many others and have been found to have the best performance when
used with encoders, resolvers and other similar components. We have over an 80%
conversion rate where we have given companies samples for evaluation and they are
compared to other precision flexible couplings.
A criterion for a good flexible shaft coupling is not only how much misalignment
it will stand, but how long the coupling will last under operating misalignment, and the
effect this misalignment has on the bearings, shaft and related components.
There are four basic types of precision couplings:
Bellows
Disc
Long-Curved Beam
Multi-overlapping Curved Beam
The following pages contain a brief general resume' of each of their capabilities.
-2-
BELLOWS TYPE COUPLINGS
One of the major problems with these couplings is their susceptibility to dents
which cause fatigue and cracking, and it is not uncommon to have them crack at the braze
points. No one can prevent assembly people from dropping a coupling.
When they are used in a straight line (perfect alignment) they have very little
windup or hysteresis. Their windup and hysteresis increase in direct ratio to the amount
of misalignment. A 1" diameter coupling has 2 minutes per ounch inch at 0.010 inch
offset (8 times ours). This is due to the bellows being compressed on one and expanded
on the other to compensate for misalignment. As torque is applied the compressed side
compresses further and puts the bellows further into a skewed position, thus the windup.
When the torque is relieved the bellows does not always come back to its original
position, and this again is dependent on the amount of misalignment.
Axial motion of the bellows type couplings is limited to the number of convolutes
and the space between those convolutes.
-3-
DISC TYPE COUPLINGS
The disc type couplings are a round shim with a large centre hole. The shims can
be phosphorous bronze, beryllium copper or thin stainless steel. There are four mounting
holes at 90° apart. Two holes 180° apart are mounted to one hub, the second set of holes
is mounted to a centre member to which a second disc is also mounted, and then finally
the other hub. It takes two discs for parallel or skewed misalignment, one disc can be
used for angular misalignment only.
This coupling should be looked at as if it were four short restrained beams held to
the hub by rivets or screws. It has the lowest windup of any coupling on the market when
in a straight line. A 1" diameter coupling is said to have two arc seconds of windup per
inch ounce of torque. When the coupling is in a misaligned condition, like all others it
has windup. Their 1.5" diameter coupling has .25 minutes per inch ounce windup at .010
compared to our .13 minutes per inch ounce (2 times ours). Other sizes are proportional.
The four short beams are less than 90° because the method of fastening takes up
part of the flexible area. As the disc bends the metal must stretch to accommodate that
bending. The greater the misalignment the more the stretching, and the longer the use the
more the stretching. With the stretching comes additional windup and hysteresis, and
also cracking at the mounting holes. Per the general manager of Renbrandt, their "life
expectancy has been increased from 160, 000 to 240, 000 cycles during last year by
changing from aluminum rivets to steel rivets. We still get cracking at that point" and the
couplings have to be replaced at that time.
When there is any axial motion, the misalignment capability is reduced because it
further stretches the four short beams and adds additional error. More significantly, it
reduces the life of the coupling.
-4-
MULTI-OVERLAPPING CURVED BEAM COUPLINGS
These couplings are constructed with three beams in each of two banks. These
beams are approximately 270° long. This coupling has less windup at its maximum
misaligned condition than any other coupling design. Further, tests have shown that it
has zero hysteresis when the shafts are free to rotate back to a zero position, such as in
the case of an encoder or resolver, and this coupling has the lowest rotary oscillation of
any of the other designs. It will take as much misalignment as other technical couplings
and is equivalent to their static torque ratings.
The short overlapping curved beams have a unique feature. When the coupling is
in a straight-lined condition all of the beams are pulling or pushing, the torque
transmission is equally divided among them, and the buckling of the beams will occur
simultaneously. When the coupling is in a misaligned condition, the buckling point of
the beams varies. As one side of the coupling expands the beams have a higher tendency
to buckle. As the other side compresses the beam is less susceptible to buckling.
Consequently, at its maximum misalignment, the negative is offset by the positive so the
buckling, torque rating, windup and all other characteristics are the same.
The torque ratings are approximately 75% of the buckling point and this is
approximately 1/2 of the breaking point. When the couplings are used within their torque
and misalignment ratings, the beams will not reach their elastic limits so theoretically
they will last forever.
-5-
HOW TO SELECT COUPLINGS
We need as much of the following information as is possible:
What is the RPM.
Determine the total torque to be transmitted through the coupling. Acceleration
and deceleration (system reverse load).
Are there any peak loads or shock loads.
What is the maximum possible misalignment.
How much torsional flexibility (windup) will be allowed or is desired.
Is there axial motion and if so, how much.
Are there unusual temperature requirements.
Check the shaft tolerances.
What method of attachment is desired. Key ways are available.
How much space is available (length and diameter).
What is the space between shafts.
NOTE: Be sure to check if there is a clamp screw orientation requirement.
Sometimes there is an access hole in a housing which must line up with the clamp
screw. We do not try to orient the screws one way or the other unless it is
specified.
-6-
MISALIGNMENT
Angular misalignment is when the extension of the centre line of two
shafts intersect. When applying a coupling the point of intersection should be
midway between the ends of the coupling, creating an obtuse angle.
Offset (parallel) misalignment is when the extension of the centre line of
two shafts do not intersect but are parallel. When applying a coupling the ends of
the shafts should be equal distance from the ends of the coupling. This will create
two obtuse angles within the coupling.
Non-intersecting angular (skewed) misalignment is when the extension of
the centre lines of the shafts are not parallel and not intersecting. When applying
a coupling the ends of the shafts should be equal distance from the ends of the
couplings.
MISALIGNMENT CAUSE AND EFFECT
The accumulated manufacturing tolerances of all the adjacent components
in a system often can cause misalignment between two shafts. Poor workmanship
or improper component alignment are most common with field replacement or
field retrofit applications.
Changes in alignment may be due to the temperature effect on varying
masses and material within an assembly; bearing wear, external loads causing
deflection, warpage of frames and/or containers.
The coupling itself can create problems. When the misalignment is
greater than the coupling can absorb, the coupling will impose a side load that can
cause shaft bending stresses and this will damage bearings. Many couplings on
the market today will wear themselves out, as well as related components in the
system, if the misalignment is not limited.
-7-
COMMON TERMINOLOGY AS APPLIED TO FLEXIBLE COUPLINGS
The following are terms understood by those who are familiar with the
rotating components and need not be defined, but should be reviewed as related to
this subject:
Axial Motion - is the movement transmitted in the direction of the centre
line of the shaft. Couplings can be used as shock absorbers to reduce this
axial motion.
Rotary Oscillation (sometimes referred to as Resonating Frequency) - is
the length of time it takes for a coupling to stop its continuous reversing
motion after a system has come to a stop.
Hysteresis - is lost motion or the amount of lag or advancement that one
hub of the coupling has in relation to the other when the coupling has
come to a stop.
Backlash - is movement of one part in relation to another. A one-piece
coupling cannot have backlash. Backlash can and does create wear, which
can increase with use. High tolerances between moving parts can be held
to reduce backlash, but this increases friction, which can increase the
torque requirements, wear the coupling and/or put stress on the related
components in the system.
Side Load (Bearing Radial Load) - is the force the coupling exerts on the
adjacent bearings perpendicular to the centre line of the shaft.
Torsion Deflection / Spring Rate / Windup / Angular Twist - are all
synonymous terms and mean the degree of rotation of one end of the
coupling in relation to the other end with a given torque applied and is
expressed in such terms as minutes per ounce inch. All shafts, ball screws
and couplings have torsional flexibility, and frequently the shaft and ball
screw have more than the coupling.
Constant Velocity - is when one end of the coupling rotates at the same
rate as the other end without respect to the misalignment between the two
ends. Not all flexible couplings are constant velocity.
Vibration and Eccentric Motion - A coupling can be used to reduce the
vibration or eccentric motion between two shafts.
Torque - characteristics vary in accordance with the system and is one of
the most important factors in applying a coupling.
-8-
Acceleration and Deceleration - are important factors to be aware of,
especially in reversing fast-stopping applications.
Shock - in a rotating system this is not uncommon. Excessive loads are
created by mechanical stops, clutches, brakes or reversing the drive. The
Rocom coupling can be designed to be a shock absorber, preventing
damage to related components in the system.
Service Factor - is the torque down rating of a flexible coupling when
under certain fixed conditions such as continuous reversing and shock
loads. This term is most frequently associated with power transmission
couplings but is sometimes asked about by potential users of instrument
couplings. The fixed down ratings are really not applicable to the market
we reach. As an example, continuous reversing can be with or without
ramp circuits, going from 0 to 10, 000 in one second or ten minutes, or just
continuously reversing and never going over 5 rmp.
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