Hydraulic Gear Pump GPM & PSI

[mpdaulerio]

Many times people request fixed displacement gear pumps by providing GPM @ a particular PSI. For instance, 45 GPM @ 3000 PSI. How is the size of the gear pump determined by this? If RPM must be provided and you use the formula RPM*CIR/231 then what does PSI have to do with the selection?

Thank you!

 

[HPost]

Gear pumps are selected based on their displacement in cubic inches or cubic centimetres / revolution.

When you know flow you want and what the rotation speed of the pump will, you can select the pump that is closest to the displacement you need. It is often the case that the exact displacement required is not available, so you would decide whether to go a little higher or lower, depending on power available and what actuator speed is required.

With all things in fluid power, as size increased, the rated pressure decreases. As size increase, the surface area on which the pressure acts increases, so the forces increase too.

On gear pumps, it is the torque on the shaft and the stress on the outlet port that dictate the max pressure. High cycle fatigue on the shaft and the body are the root causes.

There is always the option of going to stronger materials, but they are more expensive and also make the pump heavier

 

[georgeverghese]

Another parameter required for rotary pumps is the range in operating viscosity. Flowrate, dp and viscosity affect pump selection and driver power.

 

[hydtools]

All that tells me is the end result they want but don't know how to get there. Some suppliers are complicit in promoting that means of describing a pump. A lot on end-users don't know any better.

Ted

 

[lukin1977]

What does PSI have to do with the selection?
To me es pretty obvious. to me the question should be reveres. How can you select a pump with out using the working pressure?

 

[HPost]

If I went to a pump vendor, with a requirement for a single pump with say 100cc/rev displacement and a max working pressure of 200 BAR, they would just look at their standard list and offer something off the shelf.

The unit they offer will have been fully validated for use at the required pressure, with a maximum shaft speed and therefore a maximum voluemetric flow and therefore an associated maximum inlet velocity, which is related to the oil viscosity.

It may well be that this pump has an aluminium body. However, it could also be that in order to give the required life, the manufacturer has made the body from cast iron.

The point here is that a pump manufacturer will apply the material that suits the application and most, if not all gear pumps are sold as being rated for pressures up to certain value. If my application fits within that pump's working range, great. However, if my application exceeds the rated pressure, I can either accept the pump with a cast iron body, which is heavier and more expensive, or an alternative. My alternative would be to run the pump above the maximum rated load and run the risk of shaft failure or premature failure through fatigue in the pump body.

This is in the case of commodity pumps, in other words, pumps that are available off the shelf.

If I have a high volume application, where I would purchase several thousands of pumps, I could specify the flow and pressure requirement and have a pump designed and made to suit the application. This would cost many hundreds of thousands in cash as it would involve new tooling for castings and machining gear teeth and shafts and there would need to be extensive validation. I would end up with exactly what I need, but at higher cost.

Vendors that offer "off the Shelf" solutions cannot carry multiple ranges of pumps, it's just too expensive. Instead, they offer gear pumps in a particular number of groups (0 - 5 from memory). Each pump group will have a maximum rated pressure and each group will have a certain number of displacements to choose from. In general, the bigger the group, the lower the maximum working pressure.

This is because the torque on the shaft increases with flow, so the pressure has to come down. Likewise, the ports on the bigger pumps are bigger to handle the bigger flows. These bigger ports mean the stresses are higher, so again, the pressure has to be limited. Standard pumps have to fit within a standard envelope and have a standard interface, so the manufacturers can only limit the rated pressure, they cannot add material to the pump to make it stronger.

Going back to the bespoke pumps, you can make them as strong as they need to be, but they are non standard.

In summary and the main point here being that commercially available pumps are sold as commodity items, they are cheap, but you can only get what they offer. If you have money, you can get exactly what you need.

The fact still remains, all "standard" gear pumps sizes are based on the most commonly used displacements and the bigger the pump, the lower the maximum working pressure. The bigger the pump, the higher the power density and that can only grow so big before pumps become too big and/or too expensive.

The design of a pump from first principles depends on 2 things, flow and pressure. There is no way to escape that fact.