Spacer Shafts, Jack Shafts, Line Shafts, and Spool-pieces

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.

Photo 1

  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.

 Disc packPhoto 3

   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.

Share Blog Post


  1. Firouz Shabanpour on April 27, 2014 at 2:40 pm

    l want to know a bout the bent shaft and how I can find bending shaft from spectrum and analyse it.

  2. Firouz Shabanpour on April 27, 2014 at 2:44 pm

    specially for gas turbine v94/2
    thank a lot.

  3. Stan Riddle on April 28, 2014 at 9:24 am

    I would recommend contacting Siemens for specifics on vibration signature characteristics on this turbine.

  4. Luis Marques Saraiva on May 4, 2014 at 3:02 am

    I would like to know if the repeatibility in a CARDAN shaft alignment with Fixturlaser NXA Pro is always OK.
    I had some problems in a steel mill…Thanks! BR Luis

  5. Stan Riddle on May 5, 2014 at 5:43 pm

    Luis, all laser alignment tools are measuring instruments. Like all measuring instruments, if the numbers going into the calculation are the same each time, the numbers coming out of the calculation will be the same. You don’t mention how different the measurements are each time. If you are having repeatability issues while performing a cardan shaft alignment, I would check these things: (1) looseness of any brackets or bolting, especially the bolts holding the cardan bracketing, (2) external vibration, which might be changing your measurements, due to machine components moving (3) any unindended movement caused by gearing or shafting. I would recommend very careful measurement, trying to repeat the angularity values as closely as possible. I would certainly love to hear comments from anyone else.

  6. Sherif Mounir on August 14, 2014 at 8:02 am

    if there are a spacer shaft, the dimension between the the two measuring units will be divided between them.
    A= 200 mm
    D=A/2 mm
    this is right or not?

  7. Brad Case on August 28, 2014 at 11:42 am

    Sherif, you are correct if you are using an older Fixturlaser Shaft Alignment System without a spacer shaft function. The results are displayed as the angular and offset misalignment at the center of the one flexible element of the coupling. Once the “A” dimension is entered the system automatically halves “A” for the “D” dimension which is from the center of the coupling to the Moveable Sensor. (It can be changed if needed).

    The Fixturlaser NXA (and XA) Shaft Alignment Systems has a spacer shaft function which displays the coupling with two flex elements separated by the spacer or spool shaft. There is an additional dimension that is entered. The dimensions are; S to M sensor(A), spacer length, coupling center nearest the moveable machine to the M sensor(D), M sensor to front foot(B), and front foot to rear foot(C).

    Remember you can still perform a spacer shaft alignment without the spacer shaft function simply change the “D” dimension to where along the shaft you wish to view the Offset Misalignment.

Subscribe to the
Acoem USA Blog

  • This field is for validation purposes and should be left unchanged.