7 Tips for Coupling Selection

You may not always find the "perfect" coupling for your motion system, but when it comes time to select a coupling that will best suit your needs, keep these 7 tips in mind to help find the best solution possible.

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1. Inertia

Consider the inertia of your system.

The inertial mass of the rotating system has a significant impact on the demands put on your coupling. Both the rate of acceleration and rate of deceleration needs to be factored in. Sudden braking of a high-speed or high-inertia system can cause a large torque spike to pass through the coupling resulting in its damage.

2. Axial Motion

Consider operating conditions that may cause thermal expansion.

While a machine is running it may be heating up and cooling down or operating at an elevated temperature. When this happens, there can actually be some relative movement between the shafts that had not been considered previously. The reason this is important is that not all coupling solutions are capable of axial compliance, and are considered axially rigid. The risk in this situation is the possibility of putting a large axial load onto the bearings of the motor. The same motor loading can be true with thrust loads.

In a motion control system, there can be loading of the motor bearing if a solid coupling is used and transmits axial loads to the motor before engaging thrust bearings.

3. Parallel Offset

Consider the speed rating of couplings with large parallel offset.

Parallel offset is a more difficult misalignment to accommodate than angular misalignment. Couplings generally do a good job at .010 inches and below (parallel offset). Once you start hitting .020 inches or more of offset, you're going to have a harder time finding a coupling that's going to fit in a relatively compact package. If you find a coupling that has a large parallel offset capability be sure and check its speed rating. Usually when a coupling can handle a large parallel offset the product is limited in its maximum RPM rating.

5. Environment

Consider the environmental conditions your application is operating in.

Keep in mind if the coupling is operating in a harsh environment and/or corrosive atmosphere. Different materials will need to be considered depending on the type of environment. In a severely corrosive environment, specialty materials such as MP35N or Titanium may be the best option. Another environmental consideration is temperature. Certain alloys lose significant strength at elevated temperatures, or for certain elastomeric type couplings, the temperature limits may be limited. In higher temperature applications Inconel may be the material of choice. If the coupling is used in food or medical environments then the material options might be limited to certain stainless steel or Titanium.

6. RPM

Consider how balanced the coupling is for the RPM.Think about speed, RPM.

Many applications are relatively easy to solve with most couplings if you’re operating at 5,000-10,000 RPM. In today’s high-speed applications it may be possible to reach 25,000 RPM, and sometimes up into 75,000-80,000 RPM. You’ll need to factor in not just torque capacity but how well balanced is the coupling for that type of speed because if it’s not either a balanced coupling or design that’s symmetrical by nature, which makes it a statically balanced part, you’re going to be running into a vibration situation due to the imbalance.

7. Performance Rating

Consider how each manufacturer rates their couplings.

When looking for your best coupling solution, remember there is no governing body for couplings and how their performance is rated. It could be from calculations or the results of testing by the manufacturer. Make sure that you read the data carefully because one manufacturer may have a torque rating and when you look at the footnotes, their torque rating is for static torque which means it’s the torque that will yield the coupling versus another company that uses a dynamic torque, where the rating based upon a high-cycle environment. We actually de-rate the coupling torque capacity based upon realizing that they’re going to be going through millions and millions of revolutions.

The guideline to make the right coupling selection

In the power transmission field, there are many different types of couplings that can be used in order to transfer power from the gear side to the machine. Taking into consideration that each specific application has its own features, it is extremely important to analyse and check what characteristics should our coupling have, to ensure a long-life cycle and a successful performance of our machinery.

We should take into account the following factors in the coupling selection process:

1. Environmental requirements: temperature, corrosive environment, etc.
2. Accessibility: space required to apply the coupling.
3. Geometry requirements: Type of shafts.
4. Coupling sizes: maximum outside diameter and length to work with.
5. Misalignment requirements: angular, torsional misalignment, angular, etc.
6. Mechanical performance requirements: torque, speed, cushioning capability, etc.

1. Environmental requirements

It is important to analyse the area, the temperature where the coupling is going to be exposed, and also if the device is going to be in a corrosive environment or not.

2. Accessibility

The space might be an issue when placing the coupling. In some applications there is a limited or a difficult access to locate the coupling. Therefore, this is also another key aspect to take into consideration for the coupling selection.

3. Geometry requirements

The type of shafts should be checked, as well as the diameter and the length.

4. Coupling size

As an example; considering the access, space required, and the shaft types, we have to analyse what size would apply best to our installation from our range of products. 

5. Misalignment requirements

The operator should check what types of misalignments should be corrected; parallel, torsional, axial, angular or lateral.

6. Mechanical performance requirements

The main target consists on understanding what kind of performance we want to have with our coupling; high torque, high speed, repeatability or high level of flexibility and cushioning.

Decide which type of coupling will be most appropriate depending on the above aspects.

• Rigid coupling: provides a solid connection between two shafts, high precision, and torque transmission but it has no misalignment capabilities. Explained in another way, it allows no movement between two shafts. Sleeve type coupling, flange type coupling, 

• Flexible coupling: compensates for some misalignment, movement or deflection. It is very useful when having quick and short starts. Normally less torque transfer, but it can absorb misalignments and shocks.

Instructions for coupling selection

In order to determine the type of coupling to be used the following formula should be applied:

M = N hp/ n. . K

M = N kW/ n. . k

If you are looking for more details, kindly visit Flexible Coupling Types.

M = Nominal torque

N = Driving-motor power (CV or KW)

n = Minimum speed of the connected axles (rpm)

k = Multiplying co-efficient

The value obtained on applying the formula should be less or equal than the indicated in the tables of sizes and powers that refer to the corresponding coupling in the column “nominal torque”.

Notes:

The values indicated in the above table are by no means applicable to every case. If, for example, one of the machines to be coupled displays such a degree of irregularity that it is judged necessary to carry-out technical investigations of the oscillations, then it is recommended to proceed to the selection of the multiplying co-efficient using the enclosed questionnaire.

The following groups apply to the machines being driven:

K-coefficient calculation guidance for different groups of machines.

1. Continual load machines: Generators (electro genetic group). Conveyor belts. Small hoisting equipment of up to six starts per hour. Low power machinery for working wood. Small fans. Small machines of which principal movement is rotation. Small centrifugal pumps.
2. Generators (electro genetic group). Conveyor belts. Small hoisting equipment of up to six starts per hour. Low power machinery for working wood. Small fans. Small machines of which principal movement is rotation. Small centrifugal pumps.
3. Variable load machines: Small hoists. Generators. Winches. Hoisting equipment of up to 120 starts per hour. Conveyor chains. Crane movement mechanism. Sand blast equipment. Textile machinery. Transmissions. Conveyors Turbo blowers (gas blowers: compressors). Fans. Machine tools in which main movement is rotation. Large winches. Centrifugal pumps.
4. Normal size to heavy machinery: Heavy hoists. Revolving ovens Tannin barrels. Cylinder grinders. Refrigerating drums Continuous Ring Looms. Mechanical mixers. Cutters. Sharpening machines. Washing machines. Looms. Brick presses. Fans Hoisting equipment of up to 300 starts per hour. Translation mechanism.
5. Heavy machinery: Dredge control mechanism. Briquette presses. Rubber rollers. Ventilators for mines. Machinery for sand papering wood. Sand and paper grinders. Pumps with immersible piston. Cleaning drums. Machinery of oscillating movement. Compound grinders. Cement grinders. Drawbenches. Hoisting mechanisms. Hoisting Equipment of more than 300 runs per hour.
6. Heavy machinery of variable energy consumption: Large drilling installations Machinery for glossing sheets of paper. Horizontal and reciprocating vertical saws. Presses. Paper calenders. Roller trains for laminators. Drier rollers Small rollers for metals Centrifuges. Roller equipment for paper.

Example:

The elevator bucket is driven by a motor of 16 kW; n=1.450 rpm., by means of a reducer whose outlet axle rotates at a speed of n= 180 rpm. The motor and reducer are protected by a UNE-FLEX flexible axle coupling.

1. COUPLING BETWEEN MOTOR AND REDUCER

N = 16 kW
n = rpm
M = NKw/n. . k

Elevator buckets figure in group 2 of the classification. Under “variable load machinery”. The multiplying co-efficient k = 1,5 figures in the k value table under heading 2 and in the classification of machines driven by “electric motor”.

M = 16/ x x 1.5 = 158.07 Nm

Then, according to the power table. the appropriate coupling for a torque of 158 Nm. is model M-5.

2. COUPLING BETWEEN REDUCER AND ELEVATOR, BUCKET MECHANISM

N = 16 kW
n = 180 rpm
M = NKw/n. . k

M = 16/180 x x 1.5 = Nm

Then, according to the power table, the appropriate coupling for a torque of Nm is the model M-9.

Note: to carry out the correct selection of a coupling, an indication of power and speed is generally sufficient. However, it’s better to have the following information as well:

For more information, please visit Half Gear Half Rigid Couplings.

NECESSARY DATA FOR THE SELECTION OF THE APPROPIATE UNE-FLEX COUPLING DRIVEN BY ELECTRIC MOTOR