Subject: Re: Build 'em big!
> I am not sure where Tom got the figures for the pump from.
Probably used a formula like
P=V(dot) p
where P is power, V(dot) is volumetric flow rate and P is pressure.
throw in a fudge factor for pump efficiency.
The problem is the formula only works if you use a consistent set of units
( metric , use power in whats, volume flow in cubic meters per second and
pressure in pascals )
> I chose a motor (very simply) to have a higher capacity than the pump,
> so I could be sure I wouldn't strain the motor. (It was also cheap(ish)
> on ebay)
Unless you have a spare pump I would go the other way round, motors are
cheaper than pumps, and electric overload protection is fairly cheap and
simple.
> I plan to make a railcar with it. It will be double ended with one
> hydraulic "power pack" (24v dc motor + pump) driving one hydraulic
> motor.
When you say double ended do you mean a pack etc at each end ?
The railcar will carry up to 8 people (including driver), so a
> maximum load of say half a ton (maybe slightly more if I have 8 average
> adults!) ? Maximum gradient, not measured but probably 1:60 to 1:70.
8 x 70 = 560 kg, batteries 4 x 30 = 120, reservoir, pump motor 100 ? chassis
and wheels, 100 ? total weight 900 kg
Lets allow a 1:20 gradient, 2% rolling resistance, at 10 mph.
tractive force = ( 900 / 20 + 900 x 0.02 ) x 9.81 = 620 N
speed ( 10 mph ) = 4.47 m/s
delivered power = 2.8 kW ( = 3.7 Hp )
to allow for getting up to speed you will need a bit more, say 5 Hp.
If I'm right about the efficiency of hydraulic pumps and motors you would
need a very big electric motor ( 50 Hp ?? )
Anyone got figures on hydraulic efficiency ??
At 733 lbin working a 5" wheel you have 293 lbf ( = 1300 N ) available from
a single motor, but you only need about 800 N to provide reasonable
acceleration, so about 2/3 of the maximum pressure will be needed to run the
motor, or 1/3 if two motors are being used.
>
> I also plan to make two railcars and using the latest control gear from
> Cromar White, operate them in multiple if necessary.
>
> I was planning on controlling the speed of the train by controlling the
> speed of the dc motor (and thus the flow rate). Is that feasible?
>
Power electronics are quite expensive in comparison to hydraulic
restrictors, but restrictors make a system very inefficient.
When you start looking at the detail you realise why about the only vehicles
that use hydraulic drives are diggers ( they only need to move a little
every now and then, and already have a hefty hydraulic system they don't
need to use for digging when on the move ).
> Cheers
>
> Peter
>
> Jonathan Barnes wrote:
>
> > From: "puffernutter" <peter@puffer-nutter.co.uk>
> >
> >>>>The hydraulic motors I am looking at are reversible and rated at,
> >>
> >>650
> >>
> >>>>RPM @ 6.5 GPM (25 LPM), 2750 psi max. operating pressure, 733 in-lb
> >>
> >>of
> >>
> >>>>torque.
> >>>>
> >>>>The wheels are around 5", so at top RPM would give a max speed of
> >>>>9.7mph, so that's not unreasonable.
> >
> >
> >
> > I have been looking at the figures.
> >
> > The pump is a low flow high pressure design, good for rams, bad for
motors.
> >
> > the motors running together at full speed need 13 g/min =59 l.
> >
> > at the rated max pressure they would each generate 293 lb of thrust...
> > Unless this train is to be a model of an alpine rack railway.....
> >
> > What you will need is a high flow pump giving a relatively low pressure.
> >
> > What will the train's weight ( including payload ) be, and what's the
> > maximum gradient it will need to climb ?
> >
> > Hydraulic drives are very inefficient... I may not be remembering
correctly
> > but AIR, 30% for both pump and motor gave an overall efficiency of about
10%
> > on one system I speced out .
> >
> > controlling speed by controlling the pump is possible, but many motors
do
not
> > take kindly to being run slow as they rely on a shaft driven fan for
> > cooling.
> >
> > basic circuit is supply to valve from pump, double return from valve to
> > reservoir, two lines each split with a T to each motor.
> >
> > Valve needs to swap direction, with a free return as it's central
position.
> >
> > Hydraulic systems tend to cost a lot... factor in the cost of lines and
> > fittings.
> >
> > Your pump may have an integral relief valve.. if not you will need one.
> >
> >
> > Jonathan
> >
> > Barnes's theorem; for every foolproof device
> > there is a fool greater than the proof.
> >
> > To reply remove AT
Jonathan
Barnes's theorem; for every foolproof device
there is a fool greater than the proof.
To reply remove AT