Cory Danks
TOLOMATIC INC.
HAMEL, MINN.
Edited by Stephen J. Mraz
Rodless cylinders both support loads and supply guidance,
eliminating the need for other load-bearing elements
and reducing costs, size, and design time. Given
these advantages, why do designers often overlook
rodless actuators? Maybe they don’t know enough
about them and can’t spec them properly. The following
common mistakes made when sizing rodless
actuators will help engineers get the best actuator.
#1: Overestimating available air pressure
A pneumatic actuator needs the proper amount of
air pressure to perform up to specs. Engineers must
know how much air pressure is actually available. So
check your air pressure with a gage to get an accurate
reading and build in safe engineering practices. For
example, a plant may supply air at 100 psi, but pressure
can vary from site to site in the factory as much as
10% due to variable demand cycles. This means actual
available air pressure is only 90 psi. A 5 to 10% fluctuation
in air pressure is quite common and can make a
big difference in selecting the proper cylinder for an
application. It is always best to factor in a 10% pressure
loss factor from the gage air-pressure reading.
#2: Incorrectly determining the
working stroke and overall length
Part of the actuator stroke cannot be used due to
interference of internal components and the room
needed to go to the end of stroke. This is normally referred to as the actuator’s “dead length” and is determined
by the manufacturer. It should be indicated on
the dimensional information for each actuator.
To properly determine the actuator’s overall length
(OAL), you must know the distance of travel (working
stroke), then add the given dead length at each end of
the actuator. Keep in mind that adding auxiliary carriers
and other actuator options adds to the actuator’s
dead length. For example, for a cylinder with two carriers,
add the total dead length, working stroke, and
distance between the centers of carriers to determine
the cylinder’s OAL. It is important to reference the
manufacturer’s dimensional information when ordering
options to see if more dead length is needed.
#3: Under or oversizing the cylinder
When it comes to cylinder sizing, bigger is not necessarily
better. Too large a cylinder can end up costing
more in money and air consumption. On the other
hand, too small a cylinder may save a few dollars but
will not provide the best performance or operational
safety factors.
Often, a cylinder is chosen based only on the force
it produces. If a load is to be supported by the actuator,
it is important to know the bending moment capacity
of the cylinder’s bearing and load-carrying system
to determine if it can perform consistently under the load requirements. Dynamic-moment loading
should also be taken into consideration when determining
force requirements. Selecting the wrong
cylinder can lead to poor performance, reduced
life, excessive wear, and cylinder failure.
#4: Not considering resulting
moments (torques)
The position and size of the load on the cylinder
determines the bending moments applied
to the cylinder itself. For off-center or side loads,
determine the distance from the center of mass of
the load being carried to the center of the cylinder’s
carrier and calculate the resulting bending
moment.
For example, if the distance of center of mass
of the load from the center of the cylinder’s load carrying
device is 3 in., and the load is 30 lb. Then:
My = 3 in. 30 lb = 90 in.-lb
where My is the moment in the Y axis.
Mx moments (roll) are created by loads
applied at a distance from the X axis and create
a rotation around the that axis. My (pitch)
and Mz (yaw) are, similarly, moments about
the Y and Z axes, respectively. The farther a
load is from the center of the cylinder’s carrier,
the larger the resulting moment.
Published bending moments are usually maximums
and assume only one type of moment is
being applied. Some applications contain compound
moments that involve two or more of the
moments described above. Each must be evaluated
and calculated per the manufacturer’s equation to
determine if the cylinder can handle the combined
moment force.
Do I Need Shocks Or Cushions?
Consideration must be given to load position and the
resulting moments on the cylinder to determine if shock
absorbers or external load-stopping devices are needed.
In the following example, the cylinder is carrying a 10-lb
load and traveling at a final velocity of 80 ips when it hits To determine this:
Mz = Moment about Z axis
Vf = Final velocity
a = Deceleration rate
g = 386.4 ips2 (standard gravity)
s= Shock stroke
P = Load
L = distance of load from cylinder’s
load-carrying device
|
a= |
Vf2 |
= |
(80 ips)2 |
= 6,400 (ips)2 |
2s |
2 X 0.5 in. |
where a = Vf2 = (80 ips)2 = 6,400 ips2
2s 2*0.50 in (deceleration rate)
Deceleration force = a/g P = (6,400 ips2)/386.4 ips2
10 lb = 165.6 lb
Therefore, the Mz created during stopping is:
Mz = (equivalent force) L = 165.6 lb 12 in.
= 1,987.2 in.-lb
This value must be compared to the rated load capacity of
the actuator to determine proper sizing. If the moments
created during deceleration are more than the actuator’s
load capacity, you have two choices: Select a cylinder with
a larger moment rating; or put the shock absorber at the
10-lb load’s center of gravity. Putting the shock at the center
of gravity theoretically eliminates all moments on the
carrier during deceleration.
|
#5: Overlooking dynamic moment loading
Unlike rod-style cylinders, many rodless cylinders
support the load during acceleration and deceleration at each end of stroke. When there are side
or overhung loads, designers should calculate the dynamic
moments to determine which rodless cylinder
can best handle the resulting forces. Shock absorbers
(mounted on the cylinder) are normally used to help
compensate for dynamic loading’s inertial effects. In addition, it is recommended that a stopping device be
placed nearest to the center of gravity of the moving
load.
#6: Misunderstanding the relationship
between average and impact velocity
Velocity calculations for all rodless cylinders need
to differentiate between average velocity and impact
velocity. For example: Stroking a 24-in. actuator in 1 sec
yields an average velocity of 24 ips. To determine the inertial
forces for cushioning, you should know the final
or impact velocity. A reasonable guideline for calculating
the final (impact) velocity is that it’s twice the average
velocity (2*24 ips = 48 ips impact velocity).
#7: Miscalculating the cushion
or shock-absorber capacity
Most rodless actuators have internal devices that
cushion the load at end of stroke. But the final or impact
velocity must be known to determine the cylinder’s
cushioning capacities. If the final velocity cannot
be accurately determined, consider using limit switches
with valve deceleration circuits or shock absorbers.
#8: Not factoring in motion lag due to
breakaway, acceleration, and friction
It is important to understand how other forces and
losses affect the total force needed to generate the desired
motion. The total force (Ft) is the sum of acceleration
force (Fa), frictional forces (Ffr), and the breakaway
force (Fbk).
Breakaway force. It always takes a certain amount
of force to move a rodless cylinder even with no load
attached. This force is referred to as breakaway force.
When reviewing performance data for a cylinder, be
sure breakaway force is accounted for in the calculations.
In pneumatic applications, it is best to have excess
force available to assure reasonable acceleration.
Acceleration force. The force needed to accelerate
a load is typically larger than the force needed to keep
it in motion. When selecting an actuator, the cylinder’s
breakaway force and the load’s frictional drag must be
added to acceleration force requirements.
Friction forces. When two materials slide across
each other, it generates frictional force in the opposite
direction of the motion. The amount is defined by a
numeric value called the coefficient of friction (COF).
COF varies depending on the two materials and the
type of friction (sliding or rolling) generated. Engineering
reference tables list COFs for a variety of materials.
For horizontal applications, the force required to
overcome the friction is:
Ffr = μ (coefficient of friction) WL (weight of load).
#9: Vertical versus horizontal applications
Vertically mounting the cylinder brings additional
force, load, and air considerations. Vertical cylinders
need to overcome the force of gravity before they can accelerate loads upward, which means they must produce
more force than horizontal cylinders. In vertical
applications, it is best to select cylinders with twice the
force needed for adequate acceleration.
In addition, certain types of pneumatic rodless
actuators may leak air. If the actuator needs to hold a
load vertically for any length of time, the air leak can
effect how well that position is maintained.
In certain circumstances, other holding devices (such
as a brake) or external guidance system may be required
to safely control the load. Keep in mind that vertical applications
with externally guided loads still see moment
loads due to gravity. For example, a 50-lb load with a
bracket arm 12 in. from the actuator’s load carrier would
be subjected to a 600 in.-lb moment load.
Simplifying it with Software
There are many important points to consider when sizing
rodless cylinders. Knowing your available air pressure
and determining the proper working stroke and
overall length are relatively simple. But determining
the effects of moment loads, dynamic loading, inertia,
and breakaway pressure can be more complex. So some
manufacturers (Tolomatic is one) offer sizing and selection
software that make it easier to get the right cylinder.
Some programs factor in breakaway force and calculates
bending moments based on values of the speed entered.
And they use travel distance and speed values to determine
the effects of inertia on a moving load.
When using a manufacturer’s software or manually
sizing a cylinder, it is always best to discuss sizing results
and application requirements with the manufacturer.
Determining the right pneumatic rodless actuator can
be an in-depth process because there are many different
styles to consider. But in many applications, the space
saving feature and load-bearing system of rodless cylinders
make them an ideal choice for linear motion. |
#10: Underestimating the environment
Failing to factor in environmental considerations
can lead to catastrophic results. Extremely hot or cold
temperatures, external abrasives, dirty or wet conditions,
caustic fluids, and air quality are just a few
of the conditions that affect cylinder life. Frictional
wear (abrasive, pitting, adhesive and corrosive) due
to particulates or fluids hitting the cylinder can cause
premature wear or failure and increase maintenance.
Most manufacturers specify cylinder performance
based on normal operating conditions. If the cylinder
is operated in adverse environments, it is best to discuss
this with the manufacturer to determine if the
cylinder can deliver the expected performance.
Make Contact
Tolomatic Inc.
tolomatic.com