While absolutely essential on a piston-type hydraulic motor, an external case drain line is usually optional on gear motor designs–external, internal and gerotor/geroller. These types of motors don’t have a large-volume case or sump like piston motors do. But they do have internal leakage which by design, usually accumulates in a small cavity just behind the shaft seal.
By installing a high-pressure shaft seal and two check valves as shown in the schematic diagram inset above, internal leakage is drained via the return line–something that’s not acceptable for a piston motor or pump!
So the presence of a high pressure shaft seal and check valves renders a dedicated external drain line optional. But not if reliability is the primary concern. Here’s one application example from the ‘coal face’:
“Part of a design modification we carried out to our fleet of drilling rigs required two, low speed, high torque (gerotor) motors being installed on the front of the drilling rig to turn a shaft that housed wire rope brushes to clean the spoil from the drill string. The early design was without case drains because the motors did not require it according to the specification. However after several failed motors due to leaking shaft seals, we installed external case drain lines. It meant quite a bit of extra work and materials (hoses, brackets, fittings etc.) to return the drain back to tank but since then the problem has been eliminated.”
This issue is one of many apparent conflicts in hydraulics between what is OK to do-according to the component manufacturer, and what is best for optimum reliability. Of course, there will always be exceptions to the rule. You may have a gerotor motor without an external drain line that has never leaked and doesn’t look likely to (and this is most probably because the hydraulic system it’s installed on has a low and stable return line pressure).
In other words, not installing a case drain line, even when it’s optional, can turn out to be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.