Assorted Straight Bushings

Choosing the Best Linear Bushing

In our previous series, we talked about Working with Linear Bushings and different kinds of Sliding Guides in Part 1 and Part 2.  In this two-part series, we are first going to take a closer look at the two main types of bushings.  In the second article, we will discuss different drive mechanisms, and which bearings are better suited to each one.

There are three basic shapes that are available for linear ball bushings, with the main difference between them being how they are mounted: Straight, Flanged & Pillow-Block

Straight Bushings – This style of bushing does not have any mounting holes or plates on it.  It is simply a cylindrical ball bushing.  Examples of how a straight bushing can be mounted are shown below.

Straight Bushings, Mounting Method, Fixing PLates

 

Straight bushings are usually mounted using either of the following (but not limited to):

Straight bushings

 

 

 

 

 

Benefits of Straight Bushings include:

  • More freedom when designing the mounting method, as a flat surface is not required to mount a housing to
  • Cost – this style of bushing is the cheapest (thus making it the cheapest to replace)

Flanged Bushings– This style of bushing has a flange on the outer race of a straight bushing.  This is significant as it results in a more compact bushing than fitting a straight bushing into a flanged housing.  See image below.

Flanged bushings

Benefits of Flanged Bushings include:

  • Reduced manufacturing cost (no need to manufacture a housing to insert a bushing into)
  • More compact size as compared to a straight bushing in a flanged housing
  • Allows bushing to be easily mounted perpendicular to plate

Pillow-Block Bushings– This style of bushing has a straight linear bushing inserted into a pillow-block style housing.

Benefits of Pillow-Block Bushings include:

  • Reduced manufacturing cost (no need to manufacture a housing to insert a bushing into)
  • Straight bushing can be removed from housing and replaced
  • Allows bushing to be easily mounted parallel to plate

Another important factor in choosing a bushing is the length.  The longer the bushing, the more applied load and moment load it can handle (and the more expensive it is).  This is largely due to the increased number of ball bearings that are in contact with the shaft.  Bushings lengths include:

  • Single
  • Double (twice the length of a single)
  • Extended (longer than twice the length of a single)

Single length bushings have almost no moment load capacity.  As such, they are generally used in sets (for example, at the corners of the carriage of a linear motion unit).  Double and extended length bushings have a larger moment load capacity.  As such, they can be used alone.

The length of the bushings also affects clearance errors.  Specifically, given the same clearance between the shaft and the bushing, the longer bushing will have a smaller amount of angular misalignment possible then the shorter bushing.  This concept is described in the illustration below:

Single linear bushing, double linear bushing

Additionally, increasing the length of the bushing reduces the effect of swells and roughness present in the shaft, as shown below:

rail swells, rail surface roughness, double linear bushing

Think of a boat traveling through fifteen-foot ocean waves.  A thirty foot fishing vessel would follow every slope and change, while a three hundred foot ocean liner would barely feel the changing seas.

Another factor that contributes to the load capacity of the bushing is the orientation of the bushing on the shaft.  This is due to the fact that linear bushings have rows of ball bearings in them.  The orientations that allow for the least and most load capacity are shown below:

Load Capacity of Linear Bushings

To summarize, when selecting the best linear bushing for the job, first decide the most suitable mounting style for your application (straight, flanged, or pillow-block).  Next, use load rating and accuracy requirements to determine the length of the bushings that will be used.  Finally, ensure the proper orientation of the bushing when installing.  One last thing to that was not mentioned in this article that is very important for the lifespan bushing is to ensure that they are properly lubricated.

Stay tuned for the next article in the series, which will dive into different drive mechanisms and choosing the best bearing for each one.  As always, if you have any questions, feel free to comment below or email our experts at engineering@misumiusa.com.

 

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