Vehicle Overspeed prediction &

testing

Applications Engineer Training

North America Application EngineeringMark Peterson May 2006

Downhill Motor Speedup - - 2 reasons:

1. Pump becomes a motor, motor becomes a pump• Volumetric losses allow motor to go faster

than theo., rather than slower

2. Engine speed goes up to and beyond NLHI (engine is absorbing power) ~

POWER

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 ~

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 ~

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. ~

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 ~

11.85 65.84Time (Secs)

0

3500

Motor Speed (rpm)

0

3500

Pump Speed (rpm)

-8000

8000

System Delta Pressur (psid)

SCHAEF6.DAT-TimeHist.Motor_Speed_3 SCHAEF6.DAT-TimeHist.Pump_Speed_3 ...

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

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 ~