Most efficient way to rotate a cylinder using a motor?

[Will_SPHS]

This is my first post on these forums, and I was hoping someone could answer a question I have. For my high school engineering course, we spend most of the year developing a solution to a problem of our choice. We are currently developing a composting tumbler that will turn automatically several times every other day. This is to ensure the quickest rate of composting based on factors such as aeration and microbial activity. A very simple 5 minute sketch below shows the basic design. There is a backup manual turning handle as well.

My question pertains to the system that will be physically rotating the drum of the composer. What system would most efficiently rotate a drum? I have very little knowledge of rotational systems, but after some research it appears that pulleys are relatively inefficient due to slipping and stretching. That leaves me with a gear systems or a sprocket and chain system. An additional idea that I had could involve a piston (I believe they are called linear actuators?) rotating the drum (shown below), but I don't know if that would be feasible.

The drum is a hollow cylinder with relatively thins walls and a maximum weight of 600 lbs (when filled with wet dirt), although the lower density of compost would likely mean a much lower weight in the range of 300 - 350 lbs when full. There is an axle through the center of the cylinder. We are currently researching the material that is best suited to make the drum out of, but I wouldn't think that the material properties other than weight will have any impact on the turning of the drum.

I apologize since this is a very broad question and I am ignorant on the subject, but I was hoping someone could suggest what specific gear system, sprocket and chain, piston, etc. to use.

 

[Vedward]

Consider a bicycle with gears and chains. No slippage here, unless the tire skids or slips on the road. Composite rubber belts do not pose any problems, these have been used for centuries on fans and engines. There usually is an adjustable idler to adjust the belt tension. There may be some slippage when first starting, but that will stop eventually. Many fans squeal when started for a short while until they are up to speed.

I won't tell you what to do, but plastics usually will work, steel drums have been used for this. Simple calculation of power needed is weight times radius of drum will give torque needed in units of force-length. This can be used to determine motor power.

Don't over think this, pulleys work fine to turn something. Your piston could be a steam engine or gas engine or air powered piston, but is more complex than a simple motor & pulley.

 

[CWatters]

If you fill the drum with sticky material the drum may have to rotate 90 degrees before the material falls to the bottom. My point is the torque required to rotate the drum to that position might be quite high. Perhaps several hundred foot pounds depending on the radius of the drum and the mass in it? The rate of rotation needed isn't very high so that suggests you are likely to have to gear down your motor quite a lot to reduce rpm and increase torque. I suspect the ratio required might be difficult to achieve with a single stage belt or chain drive? Needs further investigation.

Perhaps try making some assumptions about the maximum torque needed at the drum. Using that and the rpm (angular velocity) work out the power needed. Find a motor with perhaps double that power and using the motors nominal rpm work out the gear ratio required.

The high torque involved may make the machine a bit of a safety hazard - capable of cutting any fingers that get trapped or pulling hair or loose clothes into the mechanism so think about guarding and safety cut-off switches.

 

[kcaldwel]

It might be worth looking at electric cement mixers, which do a very similar job. They have them at Home Depot and Lowes. You might even be able to buy a drive system as a spare from one of them. The home versions appear to be fairly low-cost gear drive systems:

http://www.homedepot.com/p/ProForce-1-2-HP-Portable-Cement-Mixer-105890DPT/202081851

There are some details of a nice enclosed gear drive system on an industrial grade unit here:

http://www.contractorsdirect.com/IMER-Minutman-Portable-Concrete-Mixer

A belt drive system could work - I think some of the old cement mixers were belt drive - but the they seem to have all gone to gear drive systems probably for low maintenance and safety reasons.

 

[Will_SPHS]

Thank you to everyone, your responses were all very helpful! It appears that we will be using a pulley or gear system. I will be doing the torque and power calculations to determine which one would work better. Also, we have definitely considered the different ways in which is would be a safety hazard, and have designed for that in our more advanced plans.

 

[Will_SPHS]

My point is the torque required to rotate the drum to that position might be quite high. Perhaps several hundred foot pounds depending on the radius of the drum and the mass in it? The rate of rotation needed isn't very high so that suggests you are likely to have to gear down your motor quite a lot to reduce rpm and increase torque. I suspect the ratio required might be difficult to achieve with a single stage belt or chain drive? Needs further investigation.

Perhaps try making some assumptions about the maximum torque needed at the drum. Using that and the rpm (angular velocity) work out the power needed. Find a motor with perhaps double that power and using the motors nominal rpm work out the gear ratio required

If the combined drum and compost when full weighs anywhere from 425 - 500 lb (depending on the moisture content and the density of the compost), with a radius of 1', then the torque would just be 500 ft*lbs. With an angular velocity of 5 rpm (.52 rad/s), the power would be approximately 250 Watts.

I was just wondering why you suggested a motor with double the calculated power. Would that most likely account for the frictional force between the drum and axle, or is that just the factor of safety? Does that mean you would suggest a motor closer to 1 HP (746 Watts) as opposed to a 1/2 HP (373 Watts), which were on the cement mixers linked by kcaldwel below you?

Edit: I guess I don't really mean a "factor of safety", but a general range of power needed above what is predicted.

 

[CWatters]

Yes just a safety margin. For a one off project the cost of over rating the motor won't be great and will expend its life. If you were going to mass produce a million or something then the cost might be more critical and it might be worth measuring the actual power required.

 

[mfb]

Startup will need more power as well.

If you need a single rotation several times per day, you can go for even lower speeds.

 

[berkeman]

Consider a bicycle with gears and chains

And using the Freewheel feature of most bicycle final drives...

https://en.wikipedia.org/wiki/Cogset#Freewheels

In most engineering optimizations, there is the opportunity to put some constraints on the final design that save a lot of cost or energy or whatever. I have several suggestions along those lines...

** I would use a low-speed geared-down electric motor to turn the drum. You don't need it turned quickly, and the large gear ratio lowers the initial motor torque requirement.

** I would use the above-mentioned free-wheel final drive arrangement (using standard bicycle components to lower cost), so that the human user can easily rotate the drum without having to disengage the geared-down electric motor assembly.

** I would make it a requirement for users who were putting new material in the drum to manually rotate the drum a few times to even out the distribution of the material some, to lower the peak torque required on the geared-down motor assembly.

So given those modifications, what peak electric motor torque do you calculate?

 

[berkeman]

Also, I would modify the drum to not be just an open cylinder. Can you picture what internal structure you could put in the drum to make it hold the material more evenly circumferentially?

 

[Will_SPHS]

Also, I would modify the drum to not be just an open cylinder. Can you picture what internal structure you could put in the drum to make it hold the material more evenly circumferentially?

We were planning on adding ridges, similar to those found in a dryer, so that the organic material wouldn't simply slide and stay at the bottom of the drum while turning. Is that what you mean, or am I talking about something else entirely? Also, does the freewheel mean that the user can simply turn the drum the opposite way of the motor without activating it or the gear system? I don't understand how they work, but if so that's really cool!

I suppose that with the lower-speed geared-down motor and with evenly distributed contents, we might be able to use a weaker (and cheaper) motor. Thank you for all your suggestions by the way!

 

[CWatters]

The manual rotation would actually be in the same direction as the motor.

 

[SongDog]

Used bicycle parts are cheap or even free at a small-town dump. You don't need high speeds, one or two rotations per day is a lot for composting. Keep the rotation rate that low and you can get by on just milliwatts with lots of mechanical advantage. Then you can use a small solar panel to drive it unattended.

 

[Will_SPHS]

Used bicycle parts are cheap or even free at a small-town dump. You don't need high speeds, one or two rotations per day is a lot for composting. Keep the rotation rate that low and you can get by on just milliwatts with lots of mechanical advantage. Then you can use a small solar panel to drive it unattended.

We were originally planning on using a solar panel to power it, but we weren't sure whether it would produce enough power. It appears though that it would be fine in that regard. The main concern I would have with it now is that we are supposed to develop a solution that would be viable on the market (including having a cost similar or cheaper to existing solutions). Although it would of course be slightly more expensive due to being automated through the use of a motor, would a solar panel significantly drive up the cost if we theoretically produced them on a larger scale? I ask this with little knowledge of solar power, but I believe earlier research led us to believe they were fairly expensive.

 

[Will_SPHS]

Also, although using a very low amount of power has been suggested several times, wouldn't I need to use a substantial base amount of power to overcome the friction between the drum and the axle? I suppose we could use bearings to decrease the friction, but there would still be a substantial amount between the two metals.

 

[CWatters]

If you use decent bearings the friction will be low compared to the torque needed to lift and turn the material. I think your earlier calculation for the power 250w wasn't far out. You could easily measure the torque needed to manually rotate the drum.

 

[mfb]

Solar powered devices won't work well without sunshine, unless you want to add a battery, but that makes the whole system more complex.

Let's say one rotation per hour. Maximal torque is ~200 kg * g *30 cm = 600 Nm, therefore the drum needs a power of ~1 W purely for lifting mass (worst case). Even if we take losses to friction into account, that's a low power.

 

[SongDog]

Solar powered devices won't work well without sunshine, unless you want to add a battery, but that makes the whole system more complex.

Let's say one rotation per hour. Maximal torque is ~200 kg * g *30 cm = 600 Nm, therefore the drum needs a power of ~1 W purely for lifting mass (worst case). Even if we take losses to friction into account, that's a low power.

Let's rather say one rotation per day. Haste makes waste. It's just a composter! Now we're looking at ~40 mW. This could easily be provided by one or two square inches of solar cell, perhaps scavenged from garden lighting. No need for a battery: if it only turns on sunny days, so what? Composting slows anyhow when there's less insolation and heating. Certainly no need of an AC extension cord, nor an outlet, nor of a massive motor.

 

[mfb]

Now we just need an electric motor for such a low power (they exist in the 10 € range), and gears that reduce speed by 4-6 orders of magnitude.

 

[berkeman]

How about using an electrically-powered mechanical clock mechanism?

 

[Will_SPHS]

Thanks everyone for all of the help, I really appreciate it! The one problem with it turning extremely slowly however, is that the door/hatch to deposit material into may be stuck in a down position when the user wants to use it. They would then have to rotate it themselves anyway, defeating the whole purpose of the automation. I basically need it to turn when the user isn't putting things in it, and in an accessible position when they want to. It is aimed towards people who are relatively weak (the elderly, etc.), or people who want to compost without having to turn it themselves (which speeds up the composting process).

The solutions then that I thought of are:

- Have it turn during the night. An electrical cord or a solar panel charging a battery would have to be used.
- Have it turn quicker (not necessarily fast, just quick enough to reduce the likelihood that a user wants access while it is turning).

 

[Will_SPHS]

How about using an electrically-powered mechanical clock mechanism?

That is actually what we have been leaning towards recently.

 

[mfb]

You could have two or three doors.

Controlling the rotation (to have exactly a full one) will need some sensors or other additional logic.

 

[SongDog]

Thanks everyone for all of the help, I really appreciate it! The one problem with it turning extremely slowly however, is that the door/hatch to deposit material into may be stuck in a down position when the user wants to use it. They would then have to rotate it themselves anyway, defeating the whole purpose of the automation. I basically need it to turn when the user isn't putting things in it, and in an accessible position when they want to. It is aimed towards people who are relatively weak (the elderly, etc.), or people who want to compost without having to turn it themselves (which speeds up the composting process).

The solutions then that I thought of are:

- Have it turn during the night. An electrical cord or a solar panel charging a battery would have to be used.
- Have it turn quicker (not necessarily fast, just quick enough to reduce the likelihood that a user wants access while it is turning).

Just tilt the drum, like a cement mixer. Even 20-30 degrees tilt should do fine. Feed it from the raised end, not the side of the cylinder. Empty with a shovel from the same hole, or tip the whole thing back to dump it.

 

[Will_SPHS]

And using the Freewheel feature of most bicycle final drives...

https://en.wikipedia.org/wiki/Cogset#Freewheels

In most engineering optimizations, there is the opportunity to put some constraints on the final design that save a lot of cost or energy or whatever. I have several suggestions along those lines...

** I would use a low-speed geared-down electric motor to turn the drum. You don't need it turned quickly, and the large gear ratio lowers the initial motor torque requirement.

** I would use the above-mentioned free-wheel final drive arrangement (using standard bicycle components to lower cost), so that the human user can easily rotate the drum without having to disengage the geared-down electric motor assembly.

I was wondering how a freewheel would attach to the motor. Would it attach to the gearbox or the axle itself? I really appreciate the suggestion by the way, we will likely be using it.

 

[David Lewis]

A scaled up grandfather clock mechanism might work. The disadvantage is that every few weeks an attendant would have to raise the weights again.