Grr! Physics problem - anyone know the answer?

You're raising 20 lbs of material every 3 seconds (6 foot run at 2 ft/sec) a height of 4 feet. Call the material weight 10 Kg (22 lbs) and the height 1.3 Meters. The energy required is E = mgh = 10Kg x 9.8 M/s^2 x 1.3 M = 127.4 Joules.

Power = Energy/Time = 127.4 Joules/3 seconds = 42.5 Watts

42.5 Watts / (746 Watts/HP) = 0.06 HP motor

Derate for margin, friction, etc.

 

I'm going to get you onto Jeopardy. I'll be your agent. We'll split the winnings evenly, 60/40.

 

I tried out once for a short-lived TV game show, I think it was called "Million Dollar Wheel of Fortune". Unfortunately I don't think I was exuberant enough and they passed.

 

I would suggest that you DO put in some sort of gearing. That way, you could buy a simpler electric motor, and the speed of incline could be changed by gear ratio. Just my two cents.

 

Geez. I would just stick a huge motor on it and say "whatever".

Who knows, maybe you could increase production by ten times.

 

now you gotta find a 0.06 hp motor!

Too bad you're not a power-mad-gear-head or you'd throw on a 1/2 hp motor from an old washing machine!

 

Have you thought this through? You're giving a 4' rise, with only 6' of travel? How do you expect the chips to stay in place? Are you planning on putting in cogs to hold them in place?

Art

 

I think he just plans to run it without cogs and watch all his items tumble!!!! LOL

Exactly what I'd have done!!! Get a 50kW motor on it, and take it from there!

 

LOL! Exactly what I was thinking, but closer to a megawatt. Where will he find 1,300HP. Maybe a TT Viper motor?!

 

That was my first thought too. Seems a bit steep. My inclination would be to get the biggest motor I could and run it well below its capability so I knew it would never be a problem in the future

-R

 

youll need a .06 hp motor


Antony


LOL

 

My head hurts from reading the answer!

 

I don't remember my physics nearly as well as ylshih does...but something doesn't compute...

Some thoughts - direct drive motors are described by certain variables/constants. You can't just pick a certain "wattage."

KV= An electric motor spins a certain amount of revolutions per volt provided to it. To find a motor that spins SOOOO slowly that you can hook it up directly, will be difficult. Most motors spin 1000-4000 rpm PER VOLT. AT least DC motors do. You will also need a regulator.

efficiency: a brushed motor has poor efficiency, likely 40% of the energy provided will be converted to heat. Sometimes much more if you are not at its peak efficiency. Brushless motors might reach 80% efficiency. This drastically effects your required "wattage."

I suspect you will need a geared motor, with a much higher wattage than whats suggested. For electric cars/planes/etc, the usual rule of thum is that 50w/lb is somewhat adequate, 100w/lb is very good.

I am comparing apples to oranges here, but I suspect 1000-2000W would serve you well, with room to spare, and not overworking the motor. Assuming 50-60% efficiency, you have 500-1000w to do the work required.

50w isn't even in the ballpark. The little motor in your pager, probably puts out 25w. I doubt 2 of those would do the work required .

 

Mike, I don't know if a 1/4 motor will be anywhere near enough. Thats about 200w of energy. Pretty damn low...really.

In reality, you are moving 20lbs 4 feet or so. BUT, just turning the conveyor will use a lot of energy. And it sounds like you intend to use it with a continuous duty cycle...

Just for fun, here is what a 100-120w electric direct drive motor looks like:

http://www.electrifly.com/motors/speed400.html

No way in hell that will do the job

 

If you were to use a 1hp /746watt capacative start single phase, 120VAC motor, ,prefferably copper wound, with an AB starter, you will be fine.
That will give you some flexability during overload and all the variables that you will encounter during it's duty cycle.
That is the important thing, the duty cycle

reliable and cheap, well the motor is less money that the AB starter.

 

I think 1 hp is about right, 1/4 hp is awefully low...

Here is the full article and calculations, for anyone so inclined:

http://www.automation.com/sitepages/pid1121.php

 

I don't know any physics (didn't do well in college when I took it), but I do know that I have a boat house. My boat house has a boat lift. My boat lift includes a steel boat cradle with 4 wooden runners on it to support the boat. The boat weighs 3500#. I have a 1hp electric motor attached to the lift that will lift the boat and cradle straight up in the air about 6 feet.

My answer is: You need less than 1hp.

 

It all depends on the gearing jim. Given high enough gearing, you could use a 1hp motor to move a house...

Think also of car hoists, etc. Tiny motor, geared 300:1, can pull your 2500# car off the ground. But its pretty slow, isn't it? If you wanted it to move at a faster rate, the power required would be substantially higher.

With a conveyor, that translates into FPM (feet per minute). If you don't mind the conveyor going very slowly, a small low wattage motor may do.

Again, once you put gearing into the picture, the answer changes dramatically. Weren't we originally talking about a direct drive motor? The original post mentioned 2fps-4fps. THATS 120-240 FPM. Thats a quick conveyor, and its flying...

 

For a sanity check, consider the following. A Porsche Boxster has a 180KW (240HP) engine, capable of accelerating a 3400 lb car (give or take with passengers) from 0-60 MPH in 6 seconds. 20 lbs is 1/170th of the weight of the car, so 180KW/170 = 1.06KW = 1.4HP would be able to accelerate 20 lbs to 60 MPH (88 ft/sec) in proportion (and that's not the top speed either). We need a LOT less than 1HP to accelerate 20 lbs to 2ft/sec.

As with any engineering problem, calculating the theoretical work done and power required is only the first step. How much more rated HP in the motor he needs than the theoretical power required goes back to the engineering part; figure out the power conversion efficiencies, frictional losses, speed range control, gearing, torque, strength of materials, design margins desired, and so forth.

The way Mike first posed his question, it was theoretical, no friction, etc. Mike can now engineer a solution and design something that does exactly what he wants with the limits that make sense and the parts available. Or if he doesn't want to go through that effort he can throw something with 5-10x the HP required into the solution and figure that he's got plenty of margin. This is the difference between engineering and "good enough", either way works.

 

Thanks Yin, thanks Bo.

I stand by my statement: You need less than 1hp.