FILLER BAR VS. WEDGE SUPPORT

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This correspondence addresses the use of filler bars versus wedges to mount riding rings (forged steel tires) on rotary equipment.

(Extrapolated from correspondence sent RSC by MDA Inc.)

Filler bars provide a number of advantages over wedges, and I will try to summarize the five most important here:

Filler bars have more bearing surface on the ID of the tire.  A filler bar is formed by rolling plate steel so the outer radius matches the inside radius of the tire.  This gives the filler bar contact along the full arc length and width of the tire.  Wedges, on the other hand, have a rectangular cross-section.  When driven in tight, the wedge makes contact along the two lines where the corners of the rectangle contact the ID of the tire.  The lines of contact between the wedge and tire induce extremely high contact stresses on the tire compared to the area of contact between a filler bar and the tire.  This situation results in more wear to the tire ID.  Furthermore, the high loading on the wedges causes them to wear, and typically require more maintenance than filler bars.

Filler bars offer more bearing surface on the shell.  Typically, filler bars cover about 60% of the circumference of the shell and extend a minimum of 4” out from each side of the tire.  It is possible to put enough wedge bodies in to cover 60% of the circumference, however, they do not extend out beyond the tire onto the shell.  The additional coverage that filler bars provide can significantly reduce the amount of bearing stress and radial stress that the shell experiences as the weight of the unit passes through to the mounting system and tire.

Filler bars allow you to mount the tire without anti-rotation bars (ARB’s).  ARB’s can be used with wedges or filler bars.  They prevent the tire from slipping around the shell during rotation of the unit.  While the ARB’s can prevent wear to the mounting components, they can also cause some catastrophic problems.  For example, if they are not welded onto the tire properly, the weld can crack and the crack can propagate into the tire.  Or, they can wear into the mating wedge or filler bar and ride up on the adjacent component (wedge or filler bar) and cause the tire to break or shell to crack.  If a wedge mounted tire does not have ARB’s, the tire will slip around and rapidly wear the wedges on the line of contact necessitating frequent wedge replacement.  With a filler bar system, the filler bars should last 5 to 7 years with a tire slipping before they need to be replaced.

You can have filler bars made at most fabrication shops that have the ability to roll plate.  Wedges require cast bodies, and the casting must match the OD of the shell and have the right length to accommodate the tire.  The wedge bodies are a specialty product with very few suppliers.  Therefore, the pricing and availability of fillers bars is extremely more favorable than wedges and bodies.

Filler bars increase the section modulus of the shell in the tire region.  Wedge bodies do not have adequate arc length to add any stiffness to the tire section of a rotary unit.  While the size, attachment to the shell, and arc length all affect how much stiffness the filler bars add to the shell section of the unit, they generally do increase the section modulus (resistance to bending). 

I do need to mention two advantages that wedges over filler bars.  First, the nature of the wedge makes radial alignment fairly simple.  Obviously, you can drive them in or back them out, as required, to obtain the desired run-out.  With filler bars, you need to use some temporary wedges and then shim the filler bar to get the proper fit.  The second advantage is the ability to change out the wedges in a short amount of time.  As the wedges wear and the tire becomes loose, if the wedge bodies (or shoes) are in good condition, the wedges can be changed out in about one shift, usually.

I hope this information helps to clear up the differences between wedges and filler bars for mounting tires.  I try to look at the comparison objectively, and I feel the filler bar system is superior to the wedge system in terms of reliability and long-term maintenance costs.

Regards,

Greg Houston, P.E.

Mechanical Design Applications Inc.