Are you sure your hydraulic machine is broken?

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On the subject of questions I get from members, there's a very valuable lesson contained in this one:

"We have some problems with a Denison hydraulic transmission pump P24S 2R1C 9A2. Oil temperature rises from 20 to 50 Celsius within 15-20 minutes of only light operation (65 bar working pressure). The pump was sent to a local workshop for repair and cost us $10,000 and yet the problem remains. The hydraulic repair shop sent an engineer to investigate but he couldn't solve it either. Any ideas?"

So the hydraulic pump wasn't the cause of the problem. Ouch! An expensive mistake. And one I've seen made all too often by hydraulic equipment users. But is there really a problem here at all?

Hydraulic oil temperature is rising to 50 Celsius within 20 minutes of operation. No big deal. Since this member is from Singapore, I would expect him to be using a VG68 hydraulic oil. Depending on the viscosity index of the particular oil he's using, optimum operating viscosity is achieved between 55 and 78 Celsius. So after 20 minutes, the system isn't even at operating temperature. Of itself, no great cause for alarm.

But why does this hydraulic system heat up so fast? Well this is a big transmission pump - 24 cubic inches or 400 cubic centimeters per rev. So it will have a big charge pump. As big as 4.8 cubic inches or 80 cubic centimeters per rev. If these pumps are turning at 1500 rpm and charge pressure is set at 23 bar, the charge pump is generating 4600 watts of pure heat load. That's equivalent to the power of two electric kettles.

And being a hydrostatic transmission, it won't have a large reservoir - perhaps as small as half the available charge pump flow per minute. Which in this case, could be as little as 16 gallons or 60 liters. So the transmission's inbuilt heating element, a.k.a charge pump, will soon warm up this volume of hydraulic oil - even with the transmission in neutral.

Based on my long experience in the hydraulics biz, the whole issue of efficiency, heat-load and cooling - and its inter-relationship with viscosity and lubrication, is grossly misunderstood by hydraulic equipment end-users. And as the above example illustrates, it can get them into a heap of trouble.

Which is why during the Breakdown Prevention seminars I present, I spend nearly half a day dealing with this whole issue. You can watch a short video clip of it, where I explain why the charge pump in a hydrostatic transmission is pure heat load - and the reason why this member blew $10,000 on an unnecessary pump overhaul. Watch the video here.

Related articles:

Hydrostatic transmissions: making sense of case drain flow - Part 1
Is this problem destroying your hydrostatic transmission?
Would you pass this simple hydraulics test?

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