Hydraulic Motor Calculations

Step 1:

Am I correct in assuming that you multiple reduction ratios for the correct

answer? Therefore 11.2 to 1 or 6.8 to 1 - Yes

2.65 x 15kph x 6.8

RPM = ---------------------- = 711 (ratio 1) or 75rad/s

0.38m

2.65 x 15kph x 11.2

RPM = ---------------------- = 1172 (ratio 2) or 123rad/s

0.38m

These are the Output Speeds at the motor, the speed at the wheel (max) is 105RPM or 11rad/s.

Step 2:

I will select 0.6 as a guess at the coefficient of grassy ground that is not

completely even.

I think you may have misread, the highest you can go is for loose sand, which is 0.3. However, a value of maybe 0.09 (average for mud) may be more suitable. (Think wet grass). Also, some of the calculations seem to be missing the gravitation constant of 9.81 to convert to a force in Newton (F=m*a). It is always a good idea to add units into your calculations for that purpose.

RR = 700kg x 0.09 x 9.81m/s^2 = 618N

Step 3:

GR = 700kg ( 0.2 + 0.09 x 0.98 ) x 9.81m/s^2 = 1979N

Step 4:

t = 10 secs

F=MxV/t

15kph x 700kg

FA = --------------------- = 291N

3.6 x 10s

Step five not required as no drawbar load

Step 6:

TE = 618N + 1979N + 291N + 0

Tractive Effort = 2888N plus 10 % for friction = say 3177 N

So now you have the tractive effort you have to translate this to power/torque required for the motor.

Step 7:

Required Torque at wheels:

3177Nx0.38m = 1207Nm

Required Power at wheels and motor (Torque x Angular Speed):

1207Nm x 11rad/s = 13277 Watt (13.3kW)

Required Torque at motor:

@ 711RPM: 13277Watt / 75rad/s = 177Nm

@ 1172RPM: 13277Watt / 123rad/s = 108Nm

This is all at 100% efficiency

Thanks for your help with the numbers. I have one of these motors:

S Series 103-2082-010 (Char-Lynn Hydraulic Motor)
Technical Specifications:
Geometric Displacement : 74 cm^3/rev (4.6 in^3/rev)
Maximum Speed : 762 rpm (continuous)
Maximum Torque : 139 N.m (continuous), 173 N.m (intermittent)
Minimum Starting Torque : 113 N.m (continuous), 146 (intermittent)
Max Pressure : 138 Bar (continuous), 172 Bar (intermittent)
Maximum Flow : 57 L/min (continuous), 76 L/min (intermittent)
Weight : 5.7 kg

How does it match with my intended use?
I don't have a pump yet, but would use a variable piston type

First of all I made a mistake in calculating the speed at ratios. Most likely only the 11.2 ratio would work at max road speed and the 6.8 ratio will most likely be used for low speed high torque conditions. The mentioned motor seems to have a max torque of 173 Nm (Intermittent) which is below the required 177 Nm. Normally the stated values are assuming 100% efficiency and in reality the efficiency can be as low as 85%. Therefore it would be better to choose a motor that has a slightly higher torque than required. Also, the maximum speed a motor can run at would be no more than the stated speed times 1.15. This motor would therefore only be able to do up to about 875 rpm continues. You should either decide on a motor with a higher speed spec or change the ratio of the diffs and/or the transfer case.

hi guys

My name is Tuan Anh , from Viet Nam , 30 years old . i have a big truoble in choosing a right hydraulic motor , i'm not sure if you can read me ? but if you do , please advice me how to calculate in matric on my e-mail : anhthultd@gmail.com . Thank you very much for your help .