You're building a soil auger, a sweeper, or maybe a forestry winch. You buy a hydraulic motor, mount it in the machine, start the pump... and complete success! The shaft spins like crazy. But the joy lasts only until the drill bit touches the ground. The motor immediately stops, and the oil howls at the relief valve.
A broken motor? A pump that's too weak? Or maybe after a moment, the rubber seal shoots out of the motor shaft and the machine is flooded with oil?
BRhydraulic experts analyze hydraulic drives down to their components. Find out how to choose a motor with immense power, and learn why one extra pipe (drainage) will save you from destroying your equipment.
1. Speed vs. Power (Why does the motor "lack power"?)
When choosing an orbital motor (popular series like BMR, BMS, BMT), customers usually look at the price and buy models with low displacement (e.g., 50 cm3 or 80 cm3). This is a fundamental mistake for machines requiring high power.
You must remember the mirror image of the principle we described for pumps:
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Oil flow (l/min) = ROTATIONAL SPEED (RPM) of the motor. The more liters of oil you pump, the faster the shaft will spin.
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Motor displacement (cm3) + Pressure (bar) = TORQUE (Power). How does it work in practice? A small motor (e.g., 50 cm3) driven by a standard pump will spin incredibly fast, but you can stop it with a proverbial "bare hand." It has no physical ability to generate high torque. If you are building a soil auger, you need a motor with high displacement (e.g., 315 cm3 or 400 cm3). It will spin slowly (e.g., 100 revolutions per minute), but its torque will easily tear through tough roots in the ground. Golden rule: For heavy work, always choose motors with greater geometric displacement!
2. The mystery of the exploding seal (Leakage line / Drain)
You bought a motor, connected the supply (A) and return (B), the machine is working, and suddenly oil is spraying around the drive shaft. You pressed the seal back into place, but the situation repeats itself. What happened?
Orbital motors have microscopic leaks inside (due to their design). Some of the pressurized oil gets into the housing (the so-called shaft chamber). If this oil has no way to escape, the pressure increases and pushes out the weakest element – the rubber shaft seal (oil seal).
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Solution: Most motors have a third, small thread on the housing (often marked on diagrams as T or Drain). This is the so-called leakage line (drain).
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If your motor operates under heavy load or the return system creates high resistance, you must connect this port with a separate hose directly, without pressure, to the oil reservoir. This will allow the "dirty" oil to drain freely from the housing, and the shaft seal will never burst again.
3. Cross-over relief valve (Shock valve) – Rescue from a twisted shaft
You've accelerated a heavy auger or a massive brush in a sweeper and suddenly release the directional control valve lever to zero. The directional control valve closes the oil flow like a concrete wall. What happens to the rotating brush?
Due to immense inertia, the brush still wants to spin. It then turns the hydraulic motor into a pump! The motor tries to pump oil but encounters a closed directional control valve. The pressure in the hose rises to critical values in a fraction of a second, which most often results in the gears inside the motor being ground down or the shaft twisting off.
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How to avoid this? Always mount a cross-over (shock) relief valve directly on the motor. When you abruptly stop the flow, this valve will gently relieve the rising pressure from one side of the motor to the other, allowing the rotating mass to slow down smoothly and safely.
Summary
Choosing a hydraulic motor is not a place for compromises. A fast motor will lack power, and the absence of a drain and safety valves will result in failure during the first heavy-duty task. If you are unsure which motor will power your auger – contact our team!
