What are shaft couplings?
Shaft couplings are mechanical components that are used to connect two rotating shafts. These components come in a lot of variation but all have the same basic functionality. Shaft couplings are used to transmit power and torque from one shaft to another. They also provide a disconnection point between the shafts for maintenance and repair. Shaft couplings also allow for misalignment between shafts. The simplest one that do not allow misalignment are similar to the shaft collars. The only difference is that they connect two shafts to transmit power. Like sleeve or muff coupling.
Why use shaft couplings?
Using shaft couplings has many benefits such as:
- Providing a disconnection point;
- Tolerating misalignment in the shafts (parallel, angular and axial);
- Reducing the shock loads between shafts;
- Altering the vibrational characteristics of rotating units;
- Protection against overloads and others…
Possible misalignment when connecting two shafts
Angular misalignment in shafts is produced when axis of two connected shafts intersect at an angle. Proper coupling selection can easily mend this is a typical misalignment. Misalignment up to 5° or even more can be fixed.
Parallel misalignment is produced when axis of shafts are parallel but are no intersecting. They are offset by a certain amount.
Axial misalignment is more rear but it can occur. This is the effect where the distance between the ends of the shafts changes during operation.
Choosing shaft couplings
When choosing the shaft couplings for your project you need to take the following things into consideration:
- Do shafts have misalignment? If there is any misalignment, parallel, angular or axial the rigid muff shaft couplings are not the best choice
Is the application motion control or power transmission?
- Motion control application are such applications where the motion of the output shaft is controlled or the position of the shaft is measured. Like in a servomotor or encoder. In these application there must not be any backlash in the movement. This backlash will cause false readings and therefore in not usable. There are shaft couplings that have zero backlash and those are a good option for this application.
- Power transmission application are applications where the main purpose is to transmit the power from on shaft to another. One shaft is usually connected to the power source. Most common applications are pumps, compressors, generators… In this cases the backlash in not important. The most important factor is the efficiency of power transmission.
- Required torque. The rule of thumb is that couplings with elastic elements can transmit less toque then couplings with all rigid elements like chain or gear connection. Whatever the case the torque rating is provided for every shaft collar by the manufacturer. Remember it is best not to choose the coupling with the maximum torque equal to the operating torque of the motor. Always choose a coupling with enough safety factor in order to be sure that the peak torque will not prematurely damage the coupling.
- Constant velocity. Constant velocity can be important in some application. Rigid couplings provide a constant velocity but if you need to manage misalignment then you can not use them. Elastic couplings do not provide a constant velocity because of the deformation of elastic elements inside them. The solution are special couplings that can deal with misalignment and also provide the constant velocity. Most common examples of these are universal joint, Thompson coupling and others.
Most typical shaft couplings in use
Jaw shaft couplings
Jaw shaft couplings are a typical coupling that has found useful applications in many areas. The coupling consists of two metal crowns that connect to both shafts and a spider that is made from elastomer. This elastomer spider is the connection between two crowns. The elastomer comes in several hardness options. The harder the elastomer spider the more torque the coupling can transmit but the dampening is lowered. In general engineering elastomer hardness of 92 Shore-A is a good compromise. Most of these couplings are backlash free so these couplings are suitable for use with servo motors. They are a bit to cumbersome to be used with encoders.
Beam shaft couplings
Beam shaft couplings are made from one piece of metal that is cut in the certain helical pattern. This is a type of flexture that connects the two shafts to transmit power. Because of their construction these type of shaft couplings do not have any backlash. They can handle a lot of angular misalignment up to 7°, and also slight axial misalignment. They cannot transmit the same amount of torque like jaw, or some other couplings. Beam couplings usually connect encoders and low powered motors.
Gear shaft couplings
Gear couplings are made out of two flanges with external gear and one sleeve with internal gear that goes over the flanges. The sleeve can be made from one or two parts that are connected together. Two component sleeves have stops on either side to stop the sleeve from disengaging with flanges. They usually transmit more power and torque. One component sleeve slides over the geared flanges and if not careful can disengage spontaneously. Lower speeds are necessary when using these couplings. Metal or plastic are the material of choice for these components. Plastic coupling improve dampening but reduce the power transmission capability.
Chain shaft couplings
Chain shaft couplings are usually used for power transmission applications. They can transmit a lot of torque, significantly more than couplings with elastic elements. The chain provides easy and fast decoupling. The couplings can have standard chains like the DIN8187 or proprietary like shown in the coupling in the picture bellow. Chain couplings can handle some angular misalingment, but not parallel or axial.
There are some interesting couplings to check out that have not made it into the list. Best examples of these are Oldham and Schmith shaft couplings. Parallel misalignment is allowed with these couplings. These couplings are in use in particular industries and are not seen every day, but when you have a particular problem to solve they can help you.