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displacement of hydraulic gear motor?

displacement of hydraulic gear motor?

displacement of hydraulic gear motor?

(OP)
Hopefully I can explain this...

I have a hydraulic gear motor, which has two spur type gears that mesh with each other inside the housing. The dimensions are:

O.D of each gear=.858" dia.
thickness of gear=.625"
# of teeth= 11 teeth per gear
distance between centers is .750"
flow= 7 gallons per minute..

The formula for finding RPM is= N=231(GPM)/displacement

How do I figure out the displacement?  I am totally stumped here...this is a motor that I have made, so I can't go to a manufacture for this..

thanks,
Brent

RE: displacement of hydraulic gear motor?

You didn't state the rpm associated with 7 gpm.

I think it will be difficult to determine the displacement without knowing the speed (or the speed without knowing the displacemnt)

That will give you gal/rev, and a simple conversion to in^3 per revolution (or any other volume unit)

Doing the math would require much more exact gear information. There are quite a few different gear profiles.

RE: displacement of hydraulic gear motor?

ROUGHLY,

there are two flow paths of dimension .625" x .108", and the length of a circle at the pitch line is 2.3562", so the displacement per revolution would be:

.625 x .108 x 2.3562 x 2 = 0.318087 in^3,

if the teeth were of zero thickness.  Assuming as a w.a.g. that the teeth occupy half the swept volume, the displacement per rev is then

0.318087 / 2 = 0.1590435 in^3.

There are 231 in^3 in a gallon, so this motor would displace

231 in^3/gal / 0.1590435 in^3/rev =  1452 -ish rev/gal

revolutions to displace a gallon.  Pushed with a flow of 7 gallons/ minute, the motor would then spin

7 gal/min * 1452 rev/gal = 10167 rev/min

...  again, ROUGHLY, and that's assuming I haven't left something out completely, which does happen.

Since that's about five to ten times the speed at which gear pumps are normally happy, I'd expect durability problems.

For a more accurate answer about the displacement per revolution, slowly pump a known volume of grease through it and count the rotations.


Mike Halloran
Pembroke Pines, FL, USA

RE: displacement of hydraulic gear motor?

For easy rough calculation of displacement per revolution you need the gear tip diameter and the gear root diameter. Work out the volume of the gear assuming it to be a cylinder (using tip dia) subtract the volume of the gear (using root diameter) Divide it by half (each gear space has a corresponding tooth) Multiply it by 2 (for the number of gears) Have used this and found it to be reasonably accurate on gear or lobe pumps, but usually a little bit bigger!

RE: displacement of hydraulic gear motor?

I have a fluid power data book and it gives a formula for calculating cubic inches displacement per shaft revolution as CIR displacement=6 x W x (2D-L) x (L-D)/2
where W=Width of gear, D=diameter of diameter of gear housing and L=Length of gear housing (for the two gears, roughly 2xgear diameter minus tooth height)

Hope this helps
KRB

RE: displacement of hydraulic gear motor?

(OP)
Thanks everyone for the replies...

As soon as I get a free second, I'll calculate and see if the #'s are reasonably accurate..

I very much appreciate the advice..

thanks,
Brent

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