Help with Design on Motorized Tool Dolly
Help with Design on Motorized Tool Dolly
(OP)
Hello Guys, this is my first post so go easy on me. I am working on a project at work, I have to design a motorized cart that can move up to a 5,000lb load. The cart is 72" long, 27" wide, and 30" tall, it has arms that hold rollers which a 8" diameter tool sits on the rollers. The cart has to move up to 3mph and the 4 wheels are 8" diameter and made of steel and roll on inverted angle iron(like a train track). Anyhow, the customer wants to see a DC motor used and have the power supply onboard. It has to have some kind of control to start slow and gain speed up to 3mph and slow down to stop. My problem is that I can not find a DC motor and gear setup to propel the cart at 3mph and handle the load. Can anyone give me any input or suggestions please? I was thinking of buying a motorized pallet jack and modify it so that I can connect it to my cart for propulsion. But obviously thats the cheap and ugly way out of my problem. I am including a picture of a cart another customer uses, but they will not give me any details to the build so I am on my own....
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RE: Help with Design on Motorized Tool Dolly
Why can you not find a DC motor and gear setup? There is lots of stuff out there.
Can you attach a cable to this thing?
How much acceleration and deceleration do you need?
Do you need to be able to make sudden emergency stops?
Are there any other safety issues?
Do you need to be cheap, or powerful and efficient?
Do you need to be able to position the table accurately? This affects torque.
JHG
RE: Help with Design on Motorized Tool Dolly
I would think that finding a DC motor would be easy. DC motors can have huge stall torques. Steel wheels on steel rails should roll easy enough that the rolling friction should be minor compared to the inertia of the cart and load. The critical question is: How fast do you have to accelerate it? A slow start means a smaller motor. I'll bet you can do the job with less than 1 hp unless they want really fast acceleration. Look at http://www.orientalmotor.com/ , they have a big selection of DC motors and controllers. Will it be powered by batteries?
Timelord
RE: Help with Design on Motorized Tool Dolly
2.) If you use v-groove wheels on both sides the tracks will need to be aligned fairly closely. Another way to accomplish this is to use v-groove wheels on one side only with flat or crowned wheels on the other. This is much more forgiving.
3.) You need to realize that the wheels are not captured and will not resist a large overturning moment.
RE: Help with Design on Motorized Tool Dolly
Friction? Regeneration? A combination?
RE: Help with Design on Motorized Tool Dolly
RE: Help with Design on Motorized Tool Dolly
If you are travelling 15ft at 3mph (4.4ft/sec), you are seeing significant acceleration. You have 7.5ft to hit 4.4ft/sec, and 7.5 to decelerate back to zero. You would be better off knowing how long your system should take to move from one position to another. 3mph may be a bit fast. Abrupt acceleration may cause something to move or tip over.
There is still lots of hardware out there. You want a reducer with a high reduction ratio and high efficiency, such as harmonic drive. There are quite a few alternatives. Forget about worm drives as they are inefficient. A large gear reduction means a smaller motor.
The engineering here does not sound complicated to me. A clear understanding of the requirements sounds much more challenging.
JHG
RE: Help with Design on Motorized Tool Dolly
I don't think this cart will have much of a duty-cycle to warrant the extra expense of more expensive reducers, so I would go with the worm-gear. As far as braking, I would look at a light-brake-torque brakemotor. You don't want a heavy brake torque to cause abrupt stopping.
RE: Help with Design on Motorized Tool Dolly
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RE: Help with Design on Motorized Tool Dolly
RE: Help with Design on Motorized Tool Dolly
Is 3mph your limit or your goal. I would say that over such a short distance it would be best to be slow.
Subway systems are powered through the rails. Would an electrified rail be too big of a safety concern? I don't know if subways are AC or DC. I would think DC but I may be wrong.
RE: Help with Design on Motorized Tool Dolly
RE: Help with Design on Motorized Tool Dolly
The RPM of an 8" wheel at 2mph is easily figured out by anyone with engineering training. Particularly since your device is battery powered, some engineering type will have to throughly analzye your drive train for torque, speed and power consumption.
Someone in-house at your end ought to know how to do this.
JHG
RE: Help with Design on Motorized Tool Dolly
The weight of your car plus its 5,000 pound payload traveling at 3 mph is more than sufficient to kill someone.
Your demonstrated inability to derive a required acceleration given top speed and distance, and to calculate wheel rpm given speed and diameter suggests that you don't have the necessary knowledge to safely design something like this.
I suggest that you hire an engineer.
RE: Help with Design on Motorized Tool Dolly
Anyone that designs anything should always keep in mind: Someone will use this and if I make a mistake someone could die. You do not want someone to die because you thought you could do something you don't know very much about.
I always have ALL of my work check by at least on other engineer even when it is not required. I do this because I never want anyone hurt or killed because I made an error.
Small error on paper equals blood in the real world.
RE: Help with Design on Motorized Tool Dolly
RE: Help with Design on Motorized Tool Dolly
RE: Help with Design on Motorized Tool Dolly
I live in a city with streetcars, which run on tracks on the road. I do not see much slippage of the metal wheels on the metal tracks. In your application, with its low speeds and accelerations, I would ignore this.
If this really matters to you, you could use a rack and pinion to move your dolly.
JHG
RE: Help with Design on Motorized Tool Dolly
Just my two bob's (20 cents) worth.
I have built many "shuttle" cars over the years carrying similar loads-first forget speed (MPH)as the main design criteria-how long do you have to move from point A to point B.
Lets say 30 seconds to move 15 Feet= 30 feet per minute=1800 feet per hour=0.34 MPH(average).
At these speeds, acceleration/deceleration issues are miniscule.
My normal method of drive is to use flange wheels (overhead crane type) with one wheel having a "ring" gear cut into the outer boss of the wheel, a pinion on the motor direct meshing with the ring gear- the other three wheels are just idlers. The gearmotor can be a flange mounted type.
Power supply to the trolley can be by overhead catenary cable, travelling busbar or by retracting cable reels.
You can use two speed brake motors-slow speed to start and stop, high speed for the majority of the travel time-changeover of speed by proximity switch sensing on start up and slowing down-or with vsd type drives with adjustable ramp up/ramp down characteristics.
All the controls can be onboard mounted which reduces the amount of cables running to the car.
Safety issues-fit a crash bar/limit switch to either end of the car, interlocked into an emergecy stop circuit to stop the car dead if it contacts an obstruction-fit fences around the total track as well.
My "rough" estimate for a drive size is 0.55 Kw at 30 fpm(friction factors are very low), but discuss the drive motor requirements with a supplier who services the overhead crane industry- this type of application is no different to a long travel crane.
Trust that this helps.
Ross
RE: Help with Design on Motorized Tool Dolly
I think you have just described a standard overhead travelling crane conguration. Do you expect any crabbing with just one driven wheel?
Regards,
Bill
RE: Help with Design on Motorized Tool Dolly
Fair comment, but all wheels are flanged and normally run on crane rail each side or can even be set up to run on the inside of tapered flange channels- this acts as guide means.
I accept that some cranes have lateral shafts with driven wheel each end (ie each side) of crane, but smaller units can have but one driven wheel-all depends on load and duty cycle/speed etc.
Ross
RE: Help with Design on Motorized Tool Dolly
You should investigate SEW Eurodrive gearmotors. I think they still make units with DC motors of sufficient power. A great resource from them is the German-made English Language Engineering Manuals which have calculation samples for this kind of thing. Very rigorous, but you will have to dig through the German English Language site to find them. SEW also had a gearmotor sizing program that was plug & chug...it may not exactly solve this design problem, but give you insight. I think you must request this through the Sales channel now so that they can chase you down and try to sell you something.
I can't say for certain, but perhaps the other manufacturers out there (Falk, Nord, Dodge, etc) have free manuals with engineering calculation sections that may be of use.
If you don't understand the math behind dynamics, then you should websearch & download the Smart Motion Cheat Sheet. Or turn this design over to a qualified mechanical engineer. You're dealing with some serious stuff here.
Also, I vaguely recall that Machinery's Handbook has some data & design guidelines on steel-on-steel rolling parameters.
TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com