April 25, 2014
Spacer Shafts, Jack Shafts, Line Shafts, and Spool-pieces
By Stan Riddle
In shaft alignment, the terms Spacer Shaft, Jack Shaft, and Spool-piece are often interchanged. They refer to a shaft which connects the driving machine to the driven machine. While the name is not so important, the type of configuration must be known, because the method of alignment can be different depending upon the type.
They can be configured in several ways:
• A short extension, which bolts rigidly to one of the machines (normally the driver), but uses a flexible coupling on the opposite end. This is most commonly referred to as a spool-piece. In this instance, the coupling center is referenced as the middle of the flexible element. The spool-piece is simply thought of as an extension.
• Two rigidly-bolted extensions, which have one or two flexible element couplings between them. These are simple extensions of the machine shafts.
• A rigid shaft, connecting the driver and driven machines by the use of two flexible couplings. This is also the typical set-up for longer spacer shafts. In this configuration, since there are two flexible elements, there are two couplings to align. It is important to note that the spacer shaft is only supported by the two flexible couplings on each end. Since this shaft is not directly supported by bearings, once the driver shaft is aligned to the driven shaft, the spacer shaft will be in alignment as well. When using dial indicators, the alignment can be completed by completing the angular (face) alignment of both couplings. Once the angular alignment of both couplings is near zero vertically and horizontally, all three shafts are in alignment. Some laser alignment tools have special programs to complete this type of alignment by measuring directly from the driven to the driven, making this type of alignment quick and accurate.
• Some longer shafts are supported by one or more bearings. In this case, the alignment is a machine train, and the spacer shaft with support bearings must be treated as machine.
• Some machines have several shafts, driving numerous machines, all coupled together and driven by a common driver. These are common referred to as line shafts. Each coupling can be aligned individually, but some laser alignment tools allow several machines to be aligned at once.
• And there’s one more. A Cardan shaft uses a spacer shaft with two cardan joints, commonly known as universal joints. Even though the driver and driven shafts are not collinear, they need to be parallel.
If you have one of these configurations, and are not sure of the correct alignment procedure, please contact VIBRALIGN, and we’ll be glad to offer some advice on the correct alignment method.