Gear Pumps & Fluid Viscosities

[bduff]

Ok guys / gals, I am currently having problems with Hyd. Fluid Viscosities effecting gear pump performance.

Background of System: Pump Charging Accumlator to 3000 psi with Pressure Switch in line to turn pump off. Accumlator Discharge every 36 seconds. Repeat Process.

Background of Problem: Over time at a 36 second discharge / recharge interval, the hydraulic oil heats up which in turn makes the oil thinner. Over time due to internal wear of the gear pump, more & more internal leakage is present. At some point (around 135 deg. F on oil temp), the pump can not create 3000 psi in the system and the pump continues to run until a preset timer is tripped. Without this timer, the pump would run forever and eventually burn up. Currently I am using 68 Weight Hydraulic Oil and replacing the pump with a new one at this point is not an option. The system currently has approx. 75000 cycles (6 months service) on it and it is now becoming a nuisance failure with approx 400 cycles before the pump can not generate the required pressure to trip the pressure switch.

What fluids do you reccomend or are there any other suggestions that you can offer?

Thanks

 

[MikeHalloran]

That pressure is at the high end of the range normally associated with gear pumps.

Given that you don't want to switch to a more appropriate pump, e.g. IMO, I'd suggest adding a big oil cooler to keep the fluid temperature down.



Mike Halloran
Pembroke Pines, FL, USA

 

[DubMac]

Really not clear on what your system is doing, but seems to be asking a lot of the pump. If the system can't be changed, then I would either try a larger pump to shorten loading time, or get a second pump and cycle between the two.

 

[hydtools]

How hot? What size pump and what speed? Why are you getting significant pump wear? Why 68 grade?
Add a cooler to keep viscosity up.
Smaller pump, smaller clearances, higher speed, lower torque at same pressure.

Ted

 

[bduff]

Thanks guys. I agree that I am asking alot of the pump and could be that I am just asking too much. I believe I will change the Top Limit Pressure to around 2500 psi and see how the system responds.

To give you a little more background, the system is operating a 2" Bore Cylinder 4" in stroke and then retracting the cylinder. And then the process starts over. In its operating environment, it must have an extension time of 0.5 sec or less (currently operates @ 0.3 sec). But in order to meet the requirement for the next cycle, the system must recharge as quickly as possible. Also, the current envelope size of the system is set and really can't get any larger (Reason for selecting a gear pump over a Vane Pump or Pressure Compensating Pump).

Also its life is outdoors and on a hot sunny day in some parts of the US can provide very hot temperatures. And most hyd oils at these temps (140 F range) are on the low end of viscosity reccomended for gear pumps.

 

[bduff]

hydtools,

Thanks for your reply. Current power unit configuration is 2HP @ 1 GPM rated for 3500psi. A cooler would definitely solve the problem but a cooler is just not an option in its enviroment. 68 grade was just something I could get my hands on quickly. I'm having a little trouble finding a supplier of higher grades of hyd. oil.

 

[hydtools]

bduff, consider lower viscosity grade. Lower viscosity will give more run time before reaching too high a temperature or too low a viscosity because less heat will be generated in the early part of the operation. Also consider a very high VI fluid so that viscosity change with temperature will be lower.

Ted

 

[LittleInch]

Try this question in the fluid power forum without double posting. There are a lot of hydraulic people there who might help.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way