[gearsoup]

I was wondering if perhaps you could give me a suggestion on a mechanism. I
have been asked to design/machine a somewhat automated spray paint system. A conveyor would carry sheets of product (plate glass, polycarbonate sheets) and I would need to adapt an arm to traverse back and forth dispensing the liquid spray. I need to devise a simple means of reciprocal motion with a stroke of roughly 18" or so. I need to keep it somewhat simple, as it will be manned by individuals who might not have great technical experience. That rules out a CNC based design, or a PLC system. Oh, it also needs to be adjustable in it's speed......

Any ideas?

[Al_The_Man]

I would look at a linear actuator, they are very common items and come in all motor type flavours.
A DC motor with a simple SCR control and limit switches to control the forward and reverse extents.
e.g. ebay 380392334380
Al.

[gearsoup]

While that would definately get me the result I need, I'm concerned that the overall size of the assembly might be too bulky. I'll keep that as a "last resort" setup, but I'd like to find something more compact. I'm thinking timing belts, pulleys, gears....
The business that is looking for this setup is rather limited on floor space.

[RICHARD ZASTROW]

Double acting air cylinder?

Dick Z

[Al_The_Man]

I'm concerned that the overall size of the assembly might be too bulky. I'll keep that as a "last resort" setup, but I'd like to find something more compact.

I would have though you could not get much more compact than a linear actuator?
Especially when compared to belts gears etc??
If you go with a double acting cylinder as Dick suggests, I would look at air over oil for a more smoother control.
Al.

[gearsoup]

Al,

Isn't a linear actuator one in-line setup? Meaning, if there is 18" of stroke, the entire work envelope is 36"? 18" for the actuator itself, then another to allow 18" of travel?

[MattTheNoob]

Look up "scotch yoke" or the drive mechanism used on shapers. The second one will allow you to have different speeds for your "application" stroke and the "return" stroke.

[Al_The_Man]

Al,

Isn't a one in-line setup? Meaning, if there is 18" of stroke, the entire work envelope is 36"? 18" for the actuator itself, then another to allow 18" of travel?

It is somewhat difficult to suggest a design with only knowing a few scant details.
If you are constrained for space then all requirements have to be known in order to recommend a design?
Al.

[gearsoup]

Al,
Lol, true. Here is the deal: Floor space is at a premium, so I'd like to keep the unit rougly inside the same footprint of the conveyor, which happens to be 2 ft wide. The conveyor length has not been determined, probably in the area of 3-4 feet. There will be a backdraft vent system in place to remove fumes and waste product.
Traverse speed of the arm is rougly 2-4 inches a second, however it would need to be adjustable outside those speeds, to allow for fine tuning.
The arm will be moving a spray gun, dispensing a liquid "paint" onto glass. The conveyor will be variable speed as well. I would set up an optical eye so the gun would only be firing when there were glass in the vicinity.
The weight of the entire arm wouldn't be that severe, as there are no cutting pressures involved, such as with a CNC router.

One note: Floor space is at a premium, but vertical is rather open. Perhaps that could be used to my advantage.........The conveyor needs to stay flat, otherwise the paint would run...

[Al_The_Man]

I could see paint over-spray a problem with both mechanism and using optical sensors?
The Scotch Yoke idea maybe an option worth looking at as Matt suggested if space is a premium?
Al.

[gearsoup]

Well, I think I've come up with a rather clever way of obtaining what I need. I'll share more as I fully develop the idea. However, I need now to find a means of powering a motor (that one is simple) stopping rotation, reversing it, stopping it, reversing it....etc. etc. Any idea on what electronics would be involved? Starting off with a DC motor, I would assume using a rheostat of some sort to control speed, then a relay switch of some sort to switch polarity, then limit switches to determine end of stroke and time to reverese direction of rotation.

[RomanLini]

I would suggest a bicycle chain and 2 small sprockets, and any simple rail and slider.

For a reversing DC motor (which sounds like a good idea) you can buy a "variable DC power supply" or "variable DC bench supply" from ebay, which will let you adjust motor speed.

You can get DC motor with worm gearbox from an auto wreckers, they are common for car windscreen wiper motors.

For reversing you need a "12 volt DPDT relay" which simply reverses the 2 motor wires to reverse the direction of the DC motor. Limit switches you can gen anywhere. The variable bench supply will help a lot to reduce motor speed and power and makes it a lot safer switching the motor from forward to reverse.

[Al_The_Man]

One danger of using a DC motor in that environment is the risk of explosion, anything electrical used in a paint booth should be TENV. or hazard free rated with explosion proof fittings.
If you can mount the motor out of the paint area, I would recommend one of the DC controllers from KB or Baldor, they can be had cheap on ebay, or not that much new.
Also for use in a paint booth, probably proximity switches are going to be the best bet.
Even electrostatic paint booths get alot of over-spray build up.
Al.

[metalfixer]

I worked in the paint shop at Chrysler Corp., and the reciprocating arms that were used in the spray booth were driven by a cam and the arm was spring loaded. You could fabricate your cam out of plywood. Ive been retired for 12 years now, and can't offer much of a description of the machine.
Larry

[RomanLini]

Thanks Al_the_man for the important safety info, I completely missed that risk of fire/explosion with the spraypainting and electrics.

Maybe the electrical part of the mechanism could be kept well away from the spray zone and just an arm or rail in the curtained area where the painting takes place? Just a thought.

[andypugh]

Given the environment, an air-over-oil rodless cylinder sounds like a good solution. Then the operators can have needle valves to adjust speed in each direction. It is probably possible to arrange the reversing mechanism so that mechanism reverses itself, so that there are no electrics at all.

Otherwise the chain/sprocket idea appeals. I think I would use a stepper, commercial drive and an Arduino. I have found that, in general, if you don't include a computer in anything electronic, you end up wishing you had, and realise after the umpteenth iteration that it would have been cheaper.

Specifically it is easy for software to switch direction if travel-limit switches are hit, reset itself and carry on. You are likely to have operators moving the head when unpowered (and maybe when powered) and recovery from that is more easily handled in software. It also makes it easy to add features (pre-programmed job speeds, or a controlled speed profile, for example).

Keeping a spare drive and pre-programmed Arduino on-hand would be a good precaution to minimise downtime.

Arduino is an expensive way to buy a microcontroller, I agree, but they are very convenient and very available.

Depending on the scale and the moving mass, you might want to go for a servo and drive rather than a stepper motor, probably using one of the step-direction input drives.

I don't know whether you have worked in this area before, but be warned that you need to make it tough. Assembly operators are not averse to deliberately breaking things to get time off...

[Joe S.]

Ok here is a purely mechanical idea, pardon the CRUDE Microsoft Paint generated drawing.
The armfollowe the track of a belt or chain running on 2 sprockets at about a 4 to 1 ratio or other ratio of your choice. As the short arm goes across the top the spray head is at it's closest to the work and you should rig it to spray at that time, as it goes across the bottom the spray head is returning to the home position, no stopping or reversal of the motor. It would work much like the oar in a rowboat. It would be completely within the space you specified using up only height.
EDIT:
I just looked up scotch yoke to see what that is, I like that better !
I never knew what that was called.

A shout out to RomanLini Glad to see you active here, Joe_S. from the MELabs forum