October 11, 2013

Broken Washers Can Cause Headaches!

By Stan Riddle


Recently a customer contacted us about increased vibration on a motor, due to a broken washer.  And they wanted to know if there was a specification for washer hardness, and a guide for bolt size to washer size.

There are many standards for bolting (USS, SAE, ASME, etc.), of which, washers are an important part.  As an example, a structure like a bridge or skyscraper may have a stringent metallurgy requirement, while one for a small industrial motor may not.  In industry, the problem is that most purchasing departments specify “washers” – without regard to strength, hardness, malleability, etc.

I have seen broken washers increase machine vibration, since it means that the bolt is now loose.  My opinion, and it is only that, is that broken washers are caused by three things:

  They are cheap

  They are improperly sized or installed, including improper sizing, excessive torque, or installing bolts much smaller than the bolt hole it goes in.

  They are deformed or worn, and should be replaced

Washers should be specified, not just purchased because they are the cheapest.  Hardened washers are available, and can be beneficial.  But increasing the hardness of metal normally increases it brittleness as well. Using two standard flat washers may help.  Your maintenance or design engineer should be able to help determine the proper application.

Washers are sized.  A 3/4″ bolt should use 3/4″ washers.  A 1″ washer fits easily on a 3/4″ bolt, but is too big to do its job, which is to:

  provide a flat surface against which the head of the bolt and the face of the washer contact.

  Increase the load area of the bolt.

  Span the hole diameter.

  Prevent damage to the machine foot by the turning bolt/nut.

 Washers should normally be installed in this fashion.

In addition, washers which are visibly damaged, deformed, cupped, or rusted should not be used.



  1. Themis K. on October 11, 2013 at 1:44 pm

    Once again a really good article with useful info!

  2. jorge on April 20, 2015 at 2:55 am

    I have a question Stan: How do I know the size of a hold down bolt in order to properly hold the motor? Is there any formula I can use to slects its diameter? Should I only select it based on the electric motor hole diameter?
    I have a case for example with a 16 mm electric motor hole using an 8 mm diameter hold down bolt.

  3. Stan Riddle on April 20, 2015 at 7:00 am

    Good question Jorge! In the US we use NEMA specs to specify motor dimensions. Most motor holes (or slots) are given as 1/32 inch increments. As an example, 27/32″ motor hole could accept a 3/4 inch bolt, and leave enough gap for alignment.

    Your example of an 8mm bolt in a 16mm hole sounds excessive. I would talk to your engineering department, or contect the motor manufacturer.