Re: HP drawn by the power steering pump.

["Arun Rao" <rao%frisbee@frisbee.pixar.com>]

On Nov 27,  4:48pm, Alan Cordeiro wrote:
> Subject: Re: HP drawn by the power steering pump.
> Hi,
>
> The discussion about pump substitutes seemed to hinge
> on the fact that I had suggested one or two horsepower
> as the load required by the pumping system.
>
> This was a WAG on my part.
>
> So I tried to come up with some scientific basis
> to see what the results would be;
>
> Area of the pistons ( 0.5" dia )  = Pi x .25 x .25 = .196 sq"
>
> Stroke of piston ( approx ) 0.2"
>
> Pressure ( 150 bar ) generated by the piston = 2200 psi
>
> WORK by EACH PISTON per stroke  = 2200 x .196 x .2  in-lbs
>                                 = 86.5 in-lbs
>                                 = 7.2 ft-lbs
>
> There are 6 pistons on the steering circuit, and two smaller
> ones on the brake circuit, so I will use 7 pistons in my calc.
>
> AT engine speeds of 1000 rpm, the pump is running at 2000 rpm
>                     = 33.3 revs/sec
>
>  Therefore, Power used = 33.3 x 7 x 7.2 ft-lbs/sec
>                        = 1680 ft-lbs/sec
>                        = 3.05 HP  (  each HP is 550 ft-lbs/sec )

> WOW, thats is a lot of load at 1000 rpm.
>
> That would work out to 9 HP at 3000 rpm, ( highway cruising
>                                           not 55 mph  )

	OK -- I'm clueless about what pistons you're talking about (I assume
	you don't mean the actuating pistons in calipers etc.).  Seems to me
	that the pump should consume very little energy once the system is
	pressurized : whatever pressure regulator valve they have in the
	circuit will open, and the pressure that the pump has to work
	against is nominally zero.

	So your calculations would be correct if the actuating pistons were
	de-pressurizing the system (accumulator?) at the same rate as the
	pump was pressurizing it, or if there was a leak in the system.  I
	suspect that it would take a lot of steering wheel turning/brake
	operation to keep the pump operating at full capacity continuously.

	While cruising, then, I would think that the significant losses might
	be the friction losses in the bearings + some small percentage of
	the pump's full capacity load.  Probably be tough to estimate the
	former, though you could probably come up with a WAG as to what the
	RMS angular displacement of the steering wheel is during straight-line
	driving (3-4 degrees, maybe?).

	OK, so it's been a while since my last M.E. course ...

	-Arun