# What NOT to do when sizing rodless cylinders

April 1, 2008
Overestimating the available air pressure is one of the most common mistakes when sizing a rodless cylinder. An insufficiency in air pressure often translates to poor actuator performance and, in some cases, malfunction.

### Mistake #1: Overestimating available air pressure

Overestimating the available air pressure is one of the most common mistakes when sizing a rodless cylinder. An insufficiency in air pressure often translates to poor actuator performance and, in some cases, malfunction. The best solution is to check available air pressure with a gauge and then build in safe engineering margins. For example, a plant may supply 100 psi of pressure, but at different locations within the plant, pressure can fluctuate as much as 10% due to variable demand cycles. This means the “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 how an actuator performs in a given application.

Lesson learned: For best results, factor in a 10% pressure loss on the measured (gauge) air pressure.

### Mistake #2: Incorrectly determining working stroke and overall length

This is a tricky one. Due to the interference of internal components and the space required to complete an entire motion cycle, a portion of a rodless actuator's stroke is useless and is often referred to as dead length. It's up to the manufacturer to determine the dead length and include this data as part of the actuator's dimensional information.

To determine the actuator's overall length (OAL), simply add the distance of travel (working stroke) to the given dead length at each end of the actuator. Note, however, that adding auxiliary carriers and other actuator options will increase the dead length. Consider, for example, a cylinder with two carriers. Here, you would add total dead length, working stroke, and the distance between the center of the carriers to determine the cylinder's OAL. It's important to reference the dimensional information provided by the manufacturer when ordering various options to determine if additional dead length is required.

Lesson learned: To determine overall length, add the working stroke (distance of travel) to the total dead length.

### Mistake #3: Under or over sizing the cylinder

When it comes to cylinder sizing, bigger is not necessarily better. In fact, “bigger” can end up costing more in terms of both money and air consumption. On the other hand, under sizing a cylinder may save a few dollars, but it will not provide optimal application performance or the appropriate safety factors. The most productive application performance is achieved when the actuator is properly sized based on load, force, and bending moment performance capacities with a safety margin factored in.

To properly size a cylinder, you need to know the following application requirements:

• Available air pressure
• Orientation of load (location relative to cylinder carrier)
• Final velocity of mass attached to carrier
• Working stroke length
• Cycle rate; cycle time
• Load's center of gravity in relation to the cylinder's load-carrying device
• Actuator orientation

Once these factors are known, it's necessary to determine the cylinder's load (thrust) force capacity based on the indicated, available air pressure. The cylinder should perform within the specified capacity range. However, if the application requires performance at the maximum level for a given cylinder, either a larger bore size should be considered or a different cylinder style with higher performance capabilities.

Once the bore size is determined, the magnitude and orientation of the load also need to be considered. Often, a cylinder is selected based only on its force capacity. But if a load is to be supported by the actuator, it's important to know the bending moment capacity of the cylinder's carrier and bearing system to determine if it's capable of performing consistently under the load requirements. Dynamic moment loading should also be taken into consideration when determining the force requirements.

Lesson learned: Selecting the wrong cylinder can result in poor performance, reduced life, excessive component wear, and cylinder failure.

### Mistake #4: Not considering the effects of resulting moments (torques)

The position and size of the load on the cylinder determines the resulting bending moments applied to the cylinder itself. Even if a load is located on and directly over the center of the carrier, it will still be subjected to bending moments on acceleration. It's important to determine if the cylinder is capable of handling the resulting moments. 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 then calculate the resulting bending moment. For example, if the distance of the load's center of mass to the center of the cylinder's carrier is 3 in. and the load being carried is 30 lb, the moment My = 3 in. × 30 lb = 90 lb-in.

Published bending moments are usually a maximum and assume only one type of moment is being applied. In certain cases, compound moments that involve two or more moments can occur.

Lesson learned: Each moment needs to be calculated per the manufacturer's equation to determine whether or not the cylinder is capable of handling the combined moment force.

Unlike rod style cylinders, many rodless cylinders must support the load during acceleration and deceleration at each end of stroke. When there are side or overhung loads, the dynamic moments must be calculated to determine which rodless cylinder is best equipped to handle the resulting forces.

Lesson learned: Shock absorbers (mounted on the cylinder) are normally used in such applications to help compensate for the inertial effects of dynamic loading. It's also recommended that a stopping device be placed nearest to the center of gravity of the moving load.

### Mistake #6: Not understanding the difference 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 in./sec. However, to properly determine the inertial forces for cushioning, it's important to know the final or “impact” velocity.

Lesson learned: A reasonable guide for determining the impact velocity is 2× the average velocity. In our example, 2 × 24 in./sec = 48 in./sec impact velocity.

### Mistake #7: Incorrectly determining cushion or shock absorber capacity

Most rodless actuators are equipped with internal devices to help cushion the load at end of stroke. Final velocity must be known to determine whether a cylinder's cushioning can withstand its impact. Load position and the resulting moments exerted on the cylinder must also be considered to determine if shock absorbers or external load stopping devices are required.

Say a cylinder is carrying a 10 lb load and traveling at a final velocity of 80 in./sec when it makes contact with the shock absorbers at the ends of the cylinder stroke. The load must be stopped within the shock absorber stroke of 0.50 in. The Mz and equivalent force applied to the cylinder's load-carrying device must be within the limits of the cylinder's rating capacities.

For our example:

vf = velocity (final)

a = deceleration rate

g = 386.4 in./sec2 (standard gravity)

s = shock stroke

Deceleration force is:

Therefore, the Mz created during stopping is:

Mz = (equivalent force) × L = 165.6 lb × 12 in. = 1,987.2 lb-in.

Lesson learned: If the final velocity can't be accurately determined, consider using limit switches along with valve deceleration circuits and/or shock absorbers.

### Mistake #8: Not factoring in the effects of motion lag due to breakaway and acceleration

It's important to understand how other forces and losses affect the total force required to produce the desired motion.

Total force calculation considering all sources of forces and frictional losses:

Ft (total force) = Fa (acceleration force) + Ffr (forces due to friction) + Fbk (breakaway force)

Acceleration force: The amount of force required to accelerate a mass is much larger than the force required to keep a load in motion. When selecting an actuator, the force required to accelerate the attached mass must be considered.

Breakaway force: Every rodless cylinder requires a certain amount of energy or force in order to move itself with no load attached. This force is referred to as the breakaway force. When reviewing the performance information for the cylinder, be sure that breakaway force is accounted for in the calculations. In pneumatic applications, it's best to have excess force available to assure reasonable acceleration is achieved.

Lesson learned: All of the forces (acceleration, breakaway, friction, etc.) must be accounted for when sizing a cylnider.

### Mistake #9: Not accounting for vertical vs. horizontal applications

When a cylinder is mounted vertically in an application, there are additional force, load, and air considerations that must be addressed. A cylinder mounted vertically needs to overcome the force of gravity first before it can accelerate a load upward. So, a vertically mounted cylinder must produce more force than a horizontally oriented cylinder.

In addition, certain types of pneumatic rodless actuators may experience some air leakage. If the actuator must hold a load vertically for any length of time, the amount of air leakage can effect how well that position can be maintained. In certain circumstances, some other type of holding device (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 experience moment loads due to the effect of gravity. For example, a 50-lb load with a bracket arm 12 in. from the actuator's load-carrying device is subjected to a 600 lb-in. moment load.

Lesson learned: In vertical applications, it's best to size a cylinder capable of twice the required horizontal force.

### Mistake #10: Underestimating environmental effects

Failing to factor in environmental considerations may lead to catastrophic results. Extremely hot or cold temperatures, external abrasives, dirty or wet conditions, caustic fluids, and poor air quality are just a few of the environmental conditions that can affect cylinder life. The effects of friction wear (abrasive, pitting, adhesive, and corrosive) from particulates or fluids on the cylinder can cause premature wear, increased maintenance, and equipment failure. Most manufacturers specify a cylinder's performance based on normal operating conditions.

Lesson learned: If the cylinder is to be operated in adverse environments, discuss this with the manufacturer to determine if the cylinder is capable of delivering the expected performance.

Special thanks to Tolomatic Inc. for this month's sizing tips. For more information, visit tolomatic.com or call (763) 478-8000.

### Mistake #4: Not considering the effects of resulting moments (torques)

The position and size of the load on the cylinder determines the resulting bending moments applied to the cylinder itself. Even if a load is located on and directly over the center of the carrier, it will still be subjected to bending moments on acceleration. It's important to determine if the cylinder is capable of handling the resulting moments. 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 then calculate the resulting bending moment. For example, if the distance of the load's center of mass to the center of the cylinder's carrier is 3 in. and the load being carried is 30 lb, the moment My = 3 in. × 30 lb = 90 lb-in.

Published bending moments are usually a maximum and assume only one type of moment is being applied. In certain cases, compound moments that involve two or more moments can occur.

Lesson learned: Each moment needs to be calculated per the manufacturer's equation to determine whether or not the cylinder is capable of handling the combined moment force.

Unlike rod style cylinders, many rodless cylinders must support the load during acceleration and deceleration at each end of stroke. When there are side or overhung loads, the dynamic moments must be calculated to determine which rodless cylinder is best equipped to handle the resulting forces.

Lesson learned: Shock absorbers (mounted on the cylinder) are normally used in such applications to help compensate for the inertial effects of dynamic loading. It's also recommended that a stopping device be placed nearest to the center of gravity of the moving load.

### Mistake #6: Not understanding the difference 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 in./sec. However, to properly determine the inertial forces for cushioning, it's important to know the final or “impact” velocity.

Lesson learned: A reasonable guide for determining the impact velocity is 2× the average velocity. In our example, 2 × 24 in./sec = 48 in./sec impact velocity.

### Mistake #7: Incorrectly determining cushion or shock absorber capacity

Most rodless actuators are equipped with internal devices to help cushion the load at end of stroke. Final velocity must be known to determine whether a cylinder's cushioning can withstand its impact. Load position and the resulting moments exerted on the cylinder must also be considered to determine if shock absorbers or external load stopping devices are required.

Say a cylinder is carrying a 10 lb load and traveling at a final velocity of 80 in./sec when it makes contact with the shock absorbers at the ends of the cylinder stroke. The load must be stopped within the shock absorber stroke of 0.50 in. The Mz and equivalent force applied to the cylinder's load-carrying device must be within the limits of the cylinder's rating capacities.

For our example:

vf = velocity (final)

a = deceleration rate

g = 386.4 in./sec2 (standard gravity)

s = shock stroke

Deceleration force is:

Therefore, the Mz created during stopping is:

Mz = (equivalent force) × L = 165.6 lb × 12 in. = 1,987.2 lb-in.

Lesson learned: If the final velocity can't be accurately determined, consider using limit switches along with valve deceleration circuits and/or shock absorbers.

### Mistake #8: Not factoring in the effects of motion lag due to breakaway and acceleration

It's important to understand how other forces and losses affect the total force required to produce the desired motion.

Total force calculation considering all sources of forces and frictional losses:

Ft (total force) = Fa (acceleration force) + Ffr (forces due to friction) + Fbk (breakaway force)

Acceleration force: The amount of force required to accelerate a mass is much larger than the force required to keep a load in motion. When selecting an actuator, the force required to accelerate the attached mass must be considered.

Breakaway force: Every rodless cylinder requires a certain amount of energy or force in order to move itself with no load attached. This force is referred to as the breakaway force. When reviewing the performance information for the cylinder, be sure that breakaway force is accounted for in the calculations. In pneumatic applications, it's best to have excess force available to assure reasonable acceleration is achieved.

Lesson learned: All of the forces (acceleration, breakaway, friction, etc.) must be accounted for when sizing a cylnider.

### Mistake #9: Not accounting for vertical vs. horizontal applications

When a cylinder is mounted vertically in an application, there are additional force, load, and air considerations that must be addressed. A cylinder mounted vertically needs to overcome the force of gravity first before it can accelerate a load upward. So, a vertically mounted cylinder must produce more force than a horizontally oriented cylinder.

In addition, certain types of pneumatic rodless actuators may experience some air leakage. If the actuator must hold a load vertically for any length of time, the amount of air leakage can effect how well that position can be maintained. In certain circumstances, some other type of holding device (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 experience moment loads due to the effect of gravity. For example, a 50-lb load with a bracket arm 12 in. from the actuator's load-carrying device is subjected to a 600 lb-in. moment load.

Lesson learned: In vertical applications, it's best to size a cylinder capable of twice the required horizontal force.

### Mistake #10: Underestimating environmental effects

Failing to factor in environmental considerations may lead to catastrophic results. Extremely hot or cold temperatures, external abrasives, dirty or wet conditions, caustic fluids, and poor air quality are just a few of the environmental conditions that can affect cylinder life. The effects of friction wear (abrasive, pitting, adhesive, and corrosive) from particulates or fluids on the cylinder can cause premature wear, increased maintenance, and equipment failure. Most manufacturers specify a cylinder's performance based on normal operating conditions.

Lesson learned: If the cylinder is to be operated in adverse environments, discuss this with the manufacturer to determine if the cylinder is capable of delivering the expected performance.

Special thanks to Tolomatic Inc. for this month's sizing tips. For more information, visit tolomatic.com or call (763) 478-8000.

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