Hydraulic Flange Connection Mistakes to Avoid
HGW Hydraulics on May 4th 2022
Many 4-bolt hydraulic flanges look similar, but small dimensional differences can matter. Code 61, Code 62, and Caterpillar-style flanges should not be interchanged unless the dimensions, pressure rating, and port requirements are confirmed.
The most common mistake is assuming that close bolt-hole alignment means the flange is acceptable. That assumption can create a serious pressure-rating mismatch.
Code 61 and Code 62 Are Different
SAE Code 61 and SAE Code 62 flanges differ in pressure range and bolt pattern. The holes may appear close, but the center-to-center dimensions are not the same.
On a 2 inch flange, one hole-center dimension may differ by only about 0.062 inch, while another dimension may be offset by about 0.75 inch. That is enough to make forced alignment unsafe.
Caterpillar-Style Flanges
Another 4-bolt style found in the field is the Caterpillar-style flange. It can look very close to an SAE Code 62 flange, but the flange head thickness is different.
An SAE Code 62 flange head may range from about 0.305 inch on a 1/2 inch flange to about 0.495 inch on a 2 inch flange. The Caterpillar-style head is about 0.560 inch for the 4-bolt flange sizes. That difference is one reason the parts should not be swapped casually.
Do Not Modify Holes to Make a Flange Fit
If the flange holes do not line up with the motor, valve, or port face, the answer is not to file or drill the flange. Forcing the connection can place a lower-rated component in a higher-pressure system or create uneven bolt loading.
A field example shows how this can happen: a mechanic installing a hydraulic motor found that a split flange did not line up with the motor housing and modified the holes. That effectively changed the connection match between a 6,000 PSI flange style and a 3,000 PSI motor pattern.
Use Matched, Rated Components
If the pressure mismatch had gone the other direction, the failure could have been obvious and severe. A flange assembly can blow off, damage the system, or injure nearby personnel when the wrong parts are forced together.
Before assembly, identify the flange code, bolt pattern, head thickness, port geometry, O-ring, pressure class, and bolt specification. A flange should fit because it is the correct matched component, not because it was modified to fit.