What are the dynamics of hydraulic motors?

[Mech101]

Hi from Sunny South Africa

I have a question about the behavior of a hydraulic motor being used as a dynamic brake (on wheels of a locomotive). Does the brake work effectively at very slow speed? I ask this because I'v seen the machine brake extremely quickly from 8 km/h, but then continue to creep forward at dead slow speed(the braking is created by hydraulic lockup in the motor when the pressure from the pump is cut). But why is the vehicle not brought to a dead stop? I've ruled out hydraulic fluid leak. I surmised that the lack of braking at very slow speed is due to the damping characteristic of a viscous fluid. Fluids and hydraulics are not my speciality, but I know a hydraulic damper cannot be compressed at high speed, but the same damper can be compressed if I press slowly. Am I on the right track? Help me along please.

 

[budt]

All commercially available hydraulic motors I am familiar with have internal bypass from one port to the other port due to opersating clearances in hteir internal components, Gears, Gerotors, Vanes, or Pistons. They can never be used to stop and hold an over running load even when new and get worse as the clearances increase due to wear.

When it is necessary to stop and hold a hydraulic motor driven device in a stationary position an external brake is required.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[Mech101]

Thanks Bud. Makes sense.
Mike