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Double motors and double pumps

Double motors and double pumps

(OP)
Hello All,

I'm working on a residential vehicle lift and just found out that I'm limited to SINGLE PHASE power. I'd originally spec'd a 10HP motor tied to our pump and estimated a ~60second lift time. Now that I'm no longer able to get a 10hp motor I'm struggling to find a way to maintain the 60seconds promised. Have 4 cylinders with about 11.6 gallons to move in 60 seconds, lift is gravity down and has an alignment system to keep the cylinders at the same stroke.

The cylinders are 3 stage telescoping. Thus it only takes about 500psi to lift the first (largest stage), but I'd like 1600+ PSI to be sure the system can make it past the last stage which only has a 2" active surface.

I'm considering a few options and tend to favor some over others but wanted to see if anyone might have some input.

1) Power two 5hp motors with single phase input VFD. Put two 5HP motors in parallel with the same pump into the circuit.
2) Try and get a 7.5 hp single phase VFD and use a TWO stage (log splitter) pump.
3) Put two 5HP motors in parallel with one high flow, low psi, and one high psi pump into the circuit.
4) Accumulator?
5) Would putting two motors/pumps in series accomplish anything?
6) Any other thoughts?

Thanks!

-Kevin

RE: Double motors and double pumps

An accumulator is the obvious answer. Then the horsepower required depends how often you have to cycle. If the cycles are infrequent you may only need a half horsepower.
I think you are dreaming if you think you can make a residential lift using 10 hp. And you need 500 psi but want 1600! When you learn what your dreams cost you will compromise on them. Accumulators are expensive and somewhat dangerous in a residential application, as are 10 hp motors and controls.

RE: Double motors and double pumps

(OP)
Thanks for the feedback!

It's not my lift. (Wish I had this problem).

It's a high end residence and we've done these before with 12hp HPUs when there's three phase power, but I'm in this situation because I'm limited to single phase power. I've already lined up at least 60A 220V supply but I can imagine getting more available if requested.

I only need 500psi at first, (since the telescoping cylinder has a larger area). The rod at the last stage has a much smaller area and my needs go up to 1600 psi.

Accumulator works well with the low duty cycle, but I, too, don't like the cost and hazard of having that sort of component in the mechanical pit. I've also never sized an accumulator and don't know what PSI and volume to acquire.

While budget is a concern, I still think I might be able to swing two moderately sized pumps side-by-side. What are the drawbacks mechanically with a dual motor option?

-Kevin

RE: Double motors and double pumps

Horsepower is horsepower. There are no advantages to two 5's versus a 10. Power is energy consumed per unit time. Energy is what is stored in an accumulator (or a battery).

RE: Double motors and double pumps

You have a few different options.

Energy-wise, a 5 hp motor should be able to whip a car around like a kite on a string.

Your suggestion of #3 above would be the best option. You will want to have two pumps on a single shaft being powered by a single motor. For the low system pressure (<2000 psi) and the likelihood that someone will never EVER do maintenance on filters, gear pumps work just fine.

Here are some gear pump stacks:

http://www.surpluscenter.com/Hydraulics/Hydraulic-...

Set up the hydraulic circuit so that the output of the high-volume pump is on an unloader circuit. This way, when the pressure is above the set point for the larger cylinders (~600 psi), you are only sending the power through the higher-pressure, lower-volume pump. Add in check valves as necessary downstream of the output of the low-volume pump and the output of the unloader circuit on the high-volume pump. Tee together the outputs of the check valves and feed this into the cylinders. That is the general operation of a log-splitter pump.

If you want to get it as fast as possible, do this with three pumps on a single shaft with the unloader circuits as explained above, taking into account the pressures encountered for each stage. A 5hp motor should EASILY move a standard size car up and down within 60 seconds.

At an overall system efficiency of 25%, a 5hp motor should be able to lift a 5,156-lb car 8 feet vertically in 60 seconds. And I think that you should be able to count on better than 25% overall system efficiency.

Engineering is not the science behind building. It is the science behind not building.

RE: Double motors and double pumps

What are you trying to lift and how high?

(6000 lbs*6 feet/60 seconds)/ (550ft-lbs/sec)/hp ~= 1 hp. Losses should not be 90%

RE: Double motors and double pumps

(OP)
More info:

It's a dual car lift with a canopy that holds a second vehicle.

Lift is rated for 12,000lb live load and has dead load of 7,500lb. Lift stroke is 90" (7.5ft).

I found a 1phase input VFD that was rated for 7.5hp. And I'm now looking at a system similar to a log-splitter with two stages. This speeds things up to under 60 seconds as long as the load stays below 9,000 lb. (Which should be any reasonable combination of two cars).

-Kevin

RE: Double motors and double pumps

I don't buy them, so you'll need to talk to a supplier - probably 2 gear pumps are cheaper than 1 variable displacement pump, but the variable displacement pump has better fine-control, is only 1 pump to plumb, and control of the lift can be customized for smoother operation across the shifting cylinder area as the cylinder telescopes. It would also get rid of the need for the VFD; the motor could run at full power and the variable pump would balance displacement to pressure so that the maximum flow was continuously available, except as mentioned, by tailoring the control for ease-in/ease-out at discontinuities in the cylinder displacement and the end points.

I'm guessing you are using a straight lift where the leverage of the cylinder is a constant 1:1 to the weight being lifted. If not, the variable displacement pump will be even more beneficial.

This sounds like a buried driveway-garage, where there's below-grade storage for one car and a second one sits on top rather than a service lift. Something like http://www.hardingsteel.com/carparx_st2.shtml

RE: Double motors and double pumps

Why are you wanting to use a VFD? If you drop the speed of an induction motor by half, you only get half of the power output. You want to exert maximum power for as much of the time as possible.

I agree completely with 3DDave - using a variable-displacement pump solves a lot of your problems. I suggested gear pumps because they are low in price and MUCH more tolerant of dirt and contamination than variable-displacement pumps. When you are running hydraulic cylinders, you have the tank being emptied and sucking in (moist) air when it does. This isn't a problem for the short term, but over prolonged years without maintenance, the gear pumps will still be running while the variable-displacement pumps may have problems.

Engineering is not the science behind building. It is the science behind not building.

RE: Double motors and double pumps

Forget about the VFD. They can be excellent devices in the right circumstances, but most commonly, their use is an indication that someone doesn't really understand what they are doing or why. This is NOT a suitable application for a VFD! All it will do in this application is increase both initial cost and energy usage.

Since this is a "high-end" residential setting, avoid external gear pumps. They are needlessly loud. Gerotor type pumps don't cost much more and are reasonably quiet.

One motor driving two fixed displacement pumps of different displacement can give the quickest lift time. Configure your system to have both pumps working in parallel for the low-pressure portion of the cycle, the larger displacement pump working alone for the intermediate pressure portion of the cycle (smaller displacement pump in recirculating or unloading mode), and the smaller displacement pump working alone for the high pressure portion of the cycle (larger displacement pump in recirculating or unloading mode). Depending on the details of the configuration of your system, lift position, sensed motor current draw, or sensed system pressure can serve to activate the control valves to activate the different pumping modes.

Spend the savings from eliminating the VFD to help incorporate excellent filtration and moisture control into the system to avoid long-term maintenance problems.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

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