April 30, 2015
By Stan Riddle
A dowel pin, in its simplest form, is a hardened steel shaft, usually very short, and can be straight or tapered. It is often used in machinery for:
- Precise locating of machinery components, such as the top half of a pump, gearbox components, line shaft bearings, etc. Dowels are usually located in diagonal corners of the machine components, to make sure machine parts line up properly.
- In high horsepower industrial machinery, dowel pins are used to help the hold down bolts control lateral, or shear movement, in machine feet. This is usually due to high torque loads on start-up.
Dowel pins, both tapered and straight, are used in some machinery installations to “pin down” machinery to prevent unintended movement. From a machinery alignment standpoint, dowel pins can be a real cause of headaches. When I started in maintenance in the early 1980’s, tapered dowel pins were placed into almost every motor, gearbox, and pump I worked on. Some of these machines were small-less than 25 horsepower, yet they were still dowelled.
Millwrights would replace a motor, align it with dial indicators, and then drive tapered pins through the motor foot and into the base plate. In almost every instance, driving the tapered pins into the base would misalign the motor. It was not as well known that the dowel pin holes should be reamed with a tapered drill, and THEN reinstall the dowel in the machine.
So, often these pins were left out. Engineers would state that the hold down bolts, if sized properly, were sufficient to overcome any shear forces created by the machine start-up. I never witnessed any machine failures due to leaving out the dowel pins.
In most general industrial applications, the use of dowel pins is not necessary. But there are some circumstances in which dowel pins are recommended, such as:
• High torque, high vibration machines such as hammer mills and crushers.
• High torque applications, such as large cooling tower gearboxes.
• High horsepower machines-typically over 1000 HP.
If dowel pins are used, engineering should be consulted as to the proper placement of the dowel locations. This is to:
• Minimize thermally-related problems, such as radial or axial thermal growth.
• Control and minimize the effects of high torque applications.
• Allow for proper maintenance, when removal or adjustment of the machine location is required.
So, what do you think of using dowel pins?
We have a few customers that will have us do an alignment on a piece of equipment. They will then drill and dowel it. Their logic is that the equipment will never need to be aligned again. Simply put back into the same location using the dowel (alignment) pins. I feel that the equipment should be checked whenever either end is removed for repair/replacement. Any thoughts?
Mike, great question! I agree with you 100%. The alignment should be re-checked each time.
Very helpfull information, thank you so much for the advice, the client meda put the pins anyways
what are the differences between spigot and dowel pin? For example, some vendors of mine said the connection on stuffing box and bearing housing in ‘in between bearing pump type’ should be using dowel pin instead of spigot. I’m not sure what’s that even mean.