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Message: <blockquote data-quote="TriHonu" data-source="post: 14537" data-attributes="member: 897">High case pressures cause the oil to press on the backside of seals causing the lips to wear a groove in the shaft and eventually the seal to totally fail.  This is pressure on the back side of the seal which is not designed to handle pressure.  Case drains are designed into pumps and motors to route this slippage/seepage oil from the back side of seals and pistons and allow the oil to return to the reservoir under minimal pressure. If you were to route this oil into the return line (which has some pressure in the line do to restrictions) you have now pressurized the back side of the seals.  This would be much worse if you were to reverse the hydraulic flow (by accident of course <img src="https://www.skidsteerforum.com/DesktopModules/AdverForums/ImageGallery/emotion-3.gif" alt="" class="fr-fic fr-dii fr-draggable " style="" />) since the return line is now the pressure line and you would now put over 3000 psi of pressure into the case drain pressurizing the back side of the seals with immediate seal blowout.In piston pumps the individual piston shafts are a rod with a ball machined on the end.  The ball is connected to a concave sleeve that fits over the ball with a disk on the top and is called the slipper pad.  Tazza has a picture of them about half way down the page on <a href="https://www.skidsteerforum.com/DesktopDefault.aspx?tabid=54&frmView=ShowPost&PostID=13406" target="_blank">This Thread (Piston from the Rotating Group)</a>.  In these types of pumps there is a small amount of oil that seeps past the piston.  It is routed to a case drain to allow it to leave the rotating group.  If it were to be trapped in the pump it will put pressure on the back of the slipper pad and will begin to hammer the slipper pad as the group rotates and can cause the slipper pads to detach from the piston or deform causing the pump to fail. Piston pumps are also sensitive to vacuum in the inlet side of the pump.  This is why the hydrostatic pump uses a charge pump to insure the pistons have sufficient inlet oil.  If there is a vacuum the slipper pad is in tension as it trys to pull oil into the pump which over time can also cause the slipper pad to detach from the piston.  This is the reason Bobcat tells you to periodically replace the filter cap on hydraulic reservoir to make sure oil can flow freely to and from the tank.  If the tank vent filter plugs you can create a vacuum in the tank causing a vacuum condition on the inlet oil.<img src="https://www.skidsteerforum.com/DesktopModules/AdverForums/ImageGallery/emotion-6.gif" alt="" class="fr-fic fr-dii fr-draggable " style="" /> TriHonu is derived from:Tri = 3 key principals the Army taught me that I apply to my life and business  1. Be technically proficient.  2. Take responsibility for your actions.  3. Keep your customers and subordinates informed.Honu is the Hawaiian name for turtle. (Turtles being natures original Hardhats and steady persistence wins the race.)TriHonu Cooperative Services is my business name.</blockquote>

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