Hydrostatic Transmission (HST) Basics

A hydrostatic transmission converts mechanical power to hydraulic power and back to mechanical power using a pump-motor combination. Unlike gear transmissions, HSTs provide infinitely variable speed ratios and can reverse direction smoothly.

Transmission Ratio

The speed ratio is determined by pump and motor displacements:

n_motor = n_pump x (D_pump / D_motor) x eta_vol

• n_motor = Motor output speed (RPM)
• n_pump = Pump input speed (RPM)
• D_pump = Pump displacement (cc/rev or in3/rev)
• D_motor = Motor displacement (cc/rev or in3/rev)
• eta_vol = Volumetric efficiency (typically 0.92-0.97)

Torque Relationship

Torque is inversely related to speed (conservation of power):

T_motor = P x D_motor / (2 x pi) x eta_mech

Or in terms of pump torque:

T_motor = T_pump x (D_motor / D_pump) x eta_overall

Power Transmission

P_out = P_in x eta_overall

Where overall efficiency = volumetric efficiency x mechanical efficiency (typically 75-90%)

Variable Displacement Operation

• Variable Pump, Fixed Motor: Speed control by varying pump displacement (0 to max). Most common configuration.
• Fixed Pump, Variable Motor: Two-speed range - reduce motor displacement for higher speed, lower torque.
• Variable Pump + Variable Motor: Wide speed and torque range. Pump controls speed in low range, motor extends high-speed range.

Efficiency Considerations

• At full displacement: Highest efficiency (85-92% overall)
• At partial displacement: Efficiency decreases due to fixed losses
• Temperature effects: Cold oil = higher volumetric loss; Hot oil = lower mechanical efficiency
• Pressure effects: Higher pressure = more leakage, lower volumetric efficiency