Comparison of Properties in 70˚ and 90˚ V Groove Guide Wheels
By Leslie Lui, Mechanical Design Engineer
The two most common V angles in V groove guide wheels are 70 and 90 degrees. The difference in V angle causes each type of wheel to have advantages and disadvantages over the other and users should select the wheel type that best meet their specific application requirements. The 70 degree groove guide wheels are better suited for applications where a low profile wheel and precise wheel-track alignment are required. The 90 degree groove guide wheels are better for applications where a small wheel diameter and high load capacity are required.
When a V groove guide wheel is assembled against its track, line contact perpendicular to the direction of travel forms between each mating set of running surfaces. Applied loads on the wheels are distributed along the lines of contact, so the length of line contact on the running surfaces must be sufficiently large to ensure the surfaces do not yield when substantial loads are applied to the wheel. This is why the grooves on the guide wheels cannot be very shallow. The running surfaces on 70 degree groove guide wheels are angled closer toward the wheels’ midplane than on 90 degree groove guide wheels. Therefore, 70 degree groove guide wheels can have shorter V heights than 90 degree wheels with equivalent line contact lengths on their respective tracks. However, 90 degree groove guide wheels will have smaller outside diameters than 70 degree groove guide wheels with equivalent line contact.
In addition to better load distribution, longer contact lengths also improve a wheel’s stability and self-alignment capability on its track. Seventy degree groove guide wheels can be better than 90 degree groove wheels with respect to these properties. The smaller V angle in the 70 degree groove wheels means larger misalignment forces have to be applied to them than to equivalent size 90 degree guide wheels to get the same amount of wheel pitch rotation and axial displacement to occur.
In any V groove guide wheel-based system, any external force applied to a wheel results in both radial and axial compressive forces between the wheel and track. The angle of the V determines how these forces are distributed in the wheel. When equal, radial compressive loads are applied to 70 degree and 90 degree V groove guide wheel and track assemblies, the resultant compressive radial load between all contacting surfaces is the same. However, the resultant compressive axial load between all contacting surfaces on the 70 degree V groove guide wheel and track assembly is higher than in the 90 degree groove guide wheel and track assembly. When the wheel contacts the track by its inner V running surfaces, any axial compressive load on the wheel’s running surfaces creates an outward bending moment in the wheel. This bending moment creates stress in the wheel, with the highest resultant stress occurring at the apex of the inner V where the two running surfaces meet. When the bending stress exceeds the wheel’s material strength the wheel can fail by cracking at the inner V apex, followed by splitting into fragments. Due to the lower resultant axial load on each running surface and the gentler angle of transition between the two running surfaces, a 90 degree wheel will have lower bending stress at the inner V apex than a similar sized 70 degree wheel subjected to the same applied radial load. Therefore, a 90 degree V groove guide wheel will support more load with less risk of the wheel splitting.
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