How Much Power?

This machine has an ageing (and very loud) 8hp Honda motor powering it. It isn't the original - traces of the old engine are on the machine, including the electric start controls in the operator's platform - something that would have been very handy to save fuel and noise. At some point in the last 36 years, the original engine died and was replaced with this standard, bare-shaft Honda engine, which unfortunately has only a pull-start. Actually, I don't really know if there's only been two engines, there may have been more during its life.

So 8hp driving 2 wheels - 4hp a side? 4hp is about 3kW so that's what's needed one each wheel in converting it to electric. Simple.

Except for a couple of factors. When moving the machine, the engine rarely sounds like it's loading down - the only time it really seems to be working hard is when hoisting the boom - my previous calcs show that needs around 2kW, or about 2.6hp. So maybe it's using a great deal less than 3kW a side.

However, one big issue is that the machine is slow - I've measured the drive shafts to the wheels spinning at 1,200rpm - with the 40:1 reduction at the wheels, that's turning the wheels at 30rpm. The wheels are about 660mm in diameter, so for every revolution they move 2m. 30rpm x 2m = 3.6km/h, which is really slow (slower than walking pace).

This makes the machine very slow to move about the orchard. Ours is a small orchard, but to move 200m takes a very tedious 3 to 4 minutes. For any distance like this or further, it's faster to hook up a ride-on mower and tow it into position.

So while it may be using a lot less than 3kW on each side at the moment, a key goal is to increase travel speed - so the question remains, how much power, and therefore, how big a motor?

Testing Torque

In an attempt to apply some physics, I decided to try and measure the torque at the input to the wheel gearbox required to move the machine. Here are my torque measurement tools:

Yes, it may look like a couple of Chinese knock-off  Vise-Grips, a piece of DIN rail and a bucket of random bits of steel, but this might just give me some idea.

The setup is with the machine on level ground, and the other drive-wheel disengaged so it free-wheels. The third castoring wheel is pointing straight ahead. Then the measurement apparatus is attached like thus:

And more random bits of steel dropped into the bucket until this happens:

With the machine creeping forward ever so slightly.

Results

Torque is mass x distance. In this case the bucket ended up weighing 0.577Kg, the rod length was .835m and  itself weighed 0.121Kg. The torque applied by the bucket is just mass times distance, so:

0.577 x 9.8 x .835 = 4.7Nm

The 9.8 turns the Kg into newtons.

For the torque exerted by the rod, one treats it as the mass of the rod at half it's length, so:

0.121 x 9.8 x 0.468 = 0.5Nm

So the left wheel needed 5.2Nm of torque to move.

However the right wheel when tested the same way only needed 3.5Nm, supported by the fact that the left drive shaft seemed a lot stiffer to turn. I'll have to look into that - I previously replaced the axle shaft on the left gearbox, and am now wondering whether I buggered up a bearing position or something, causing binding.

Torque into Power

Ignoring the strange result for the left wheel, I can work on the basis that the machine needs 3.5Nm of torque to move forward. Digging up a  formula for power:

Power (kW) = torque x 2pi x rpm / 60,000

Plugging in our numbers, including the current drive shaft speed:

Power = 3.5 x 2pi x 1,200 / 60,000 = 0.44kW

Which frankly isn't a whole heap, making me think my torque measurement might be a bit flaky.

Forging on, though, as mentioned before, the machine is too slow. I'd like it to go 3 times as fast, so I want 1.3kW. That's 1.7hp. Still  not a great deal of power...

However, this is for the machine on the flat, the castor wheel pointing in the direction of travel. In many cases this machine will be on a slope, the platform raised and only one wheel driven in order to turn the machine, and quite possibly with the castoring wheel turned square to the way it needs to be. In that situation the torque requirement will be far greater, although the speed in that situation will be far less than needed for travelling to a new spot.

Conclusion

I'm not really sure where that leaves things. I certainly need to work out why one wheel needs almost twice the torque compared to the other, but I'm really not sure if a machine with an 8hp engine really only needs 440 watts of power (2/3hp) to move. Perhaps I need to try a different method for torque measurement...

The fact is that I've already semi-decided on what motors to use, and they are rated at 5kW continuous (15kW peak), so with one on each wheel, that's 10kW. Apart from the extra cost of the oversize motors, the other consideration is that if they end up operating at very low power, it's likely they'll be below their maximum efficiency, so I won't be getting the best bang for buck out of my batteries...

While I ponder that, I'll try and work out what's up with that left wheel gearbox.