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Vehicle Overspeed prediction & testing Applications Engineer Training

Vehicle Overspeed prediction & testing Applications Engineer Training. North America Application Engineering Mark Peterson May 2006. POWER. Downhill Motor Speedup - - 2 reasons:. Pump becomes a motor, motor becomes a pump

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Vehicle Overspeed prediction & testing Applications Engineer Training

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  1. Vehicle Overspeed prediction & testing Applications Engineer Training North America Application Engineering Mark Peterson May 2006

  2. POWER Downhill Motor Speedup - - 2 reasons: • Pump becomes a motor, motor becomes a pump • Volumetric losses allow motor to go faster than theo., rather than slower • Engine speed goes up to and beyond NLHI (engine is absorbing power)~

  3. Downhill Motor Speedup - - other considerations: • Newer engines have less braking power capability • Speeds up more on the same downhill grade • Know your terminology: • “Brake HP” does not equal “braking power capacity”!! • Many times engine mfrs. don’t have good data on braking power capacity • Engine speed “Flare up” is very difficult (impossible?) to predict before vehicle testing • 25% overspeed not uncommon on new engines • Engines can handle much more overspeed without failure than our pumps & motors can! • Just because the engine guys aren’t worried doesn’t mean we shouldn’t be ~

  4. Pump Overspeed During Deceleration (flat ground) • Hydr. Motor won’t speed up, but pump may overspeed • Pump destrokes, becomes smaller, faster “motor” • Heavy vehicles when decelerating • If pump destrokes too fast, can overpower engine • Can control this with ramp rates or orifices ~

  5. Highest Risk Vehicles • Relatively heavy machines with relatively small engines are most at risk • Especially if they transport at high speed • Very little engine braking capability + high weight + high speed • Ag Sprayers, Combines, Railway maintenance machines etc. ~

  6. Downhill Testing - - How-To: • Test in “real” downhill conditions • May need to challenge your OEM to find a safe downhill site for testing • Apply at least max. vehicle weight (drawbar machines pulling loaded trailers, etc.) • Monitor Motor & Pump speeds • Do sustained full speed driving down steep hill • Also do deceleration down steep hill (check for engine & pump O/S) • Need safe test site - - Assume the vehicle will lose braking ability ~

  7. Downhill Testing - - How-To: • Worst case for “block tipping” is usually just after cresting a hill • Near highest speed condition • Both system pressures near charge • Very little holddown force

  8. What to do? • Select motor min. angle to avoid the “point of no return” • Use microcontroller based over speed control • Select ramp rates to avoid engine over speed during braking • In extreme cases, OEM can add braking capabilitiy: • Based on speed, microcontroller can: • Actuate engine exhaust brakes (“Jake brake”) • Add a hydraulic load (gear pump over orifice) • Apply vehicle service (not parking!) brakes • Theoretically - - not sure it’s been done before ~

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