June 17, 2012
Shaft Alignment with a Fluid Coupling
By Steve Matthews
Last week, a customer asked me how to do a laser alignment on a fluid coupling. This question comes up occasionally, so I thought it would be a good subject to address here on thealignmentblog.com. First, it would be helpful to define “fluid coupling.” Fluid couplings transmit power from a driving shaft to a driven shaft on the principle of hydrodynamic power transmission–the shafts are not connected mechanically. In fluid couplings, two bladed wheels face each other: a pump wheel on the driving side, and a turbine wheel on the driven side. The cavity or “working circuit” is filled with a fluid. The rotation of the pump wheel causes the movement of the fluid which drives the turbine wheel. These coupling types are referred to by many terms such as fluid couplings, fluid drives, fluid drive couplings, hydrodynamic couplings, hydraulic couplings, clutch couplings, etc.
Fluid couplings have many uses in a variety of industries. For the purpose of a shaft alignment discussion, we will focus on two basic types. Type 1 in which the shell is supported only by the shafts, and Type 2 in which the shell is supported by a set of bearings in a housing.
For fluid couplings supported by their own shafts (Type 1 picture), the shaft alignment application is a standard horizontal alignment of two shafts. The challenge is fixturing. As seen in the picture, the very simple solution is to use extension rods so that the laser alignment system sensors extend far enough above the fluid coupling shell to take measurements. Magnetic fixtures attaching to the coupling (shell) generally do not work for these applications. In this picture, submitted by a customer, you can also see that they use a magnetic base as a steady rest for the laser heads. This alignment is often done in the same way you would do an “uncoupled” alignment because the shafts will rotate independently. Remember, with fluid couplings there is no mechanical connection between the shafts. The customer who submitted this photo does this alignment as a coupled alignment because they have a pin or bolt that is used to connect the two shafts so that they rotate together.
Fluid couplings supported by bearings (Type 2 picture) are a different application. In this case, the fluid coupling is considered an independent machine element. It will be connected to the driver with a mechanical coupling, and to the driven with a mechanical coupling. Therefore, the fluid coupling itself is not the alignment to be done. The alignment is from the driver to the fluid coupling input shaft, and from fluid coupling output shaft to the driven machine. This can be done as a machine train application but more likely it would be done in sequence – first aligning the fluid coupling to the driven machine (possibly a gearbox), then next aligning the driving machine (Motor) to the fluid coupling drive.
Precision alignment is particularly important for fluid coupling machinery for different reasons than mechanical couplings. Proper alignment is imperative to ensure performance (transmission of rotational speed), and to ensure proper sealing so that the hydraulic fluid does not leak from the shell.
Do you take any special steps when a fluid coupling is involved in your shaft alignment job?