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08-21-2011, 12:25 PM #1
Boston Gear unit fails, question of maximum weight.
I have a machine I have had to replace this gear unit on three times now'
The brass gear that rides on the worm gear just gets pummeled.
If the gear unit is rated at 527 lbs torque, How much weight can it lift, or should it lift. The cam rides on a 1.5 inch ball bearing.
I plan to add air cylinders on each side of the head this cam is lifting so I won't have to replace it again.
08-21-2011, 12:59 PM #2
Hi i see your into the printing buisseness had a couple of these come into the shop before look for a local gear machine shop in get one made from steel and send it out for hardning to just below the worm drive hardnes.
this will last longer but when it lets go there will need to have a new box ready.
08-21-2011, 01:11 PM #3
"527lbs torque" isn't torque its a weight or force. We need to know if that is inch lbs or ft lbs. To find the maximum lift divide the torque rating by the number of inches for an inch lb rating or feet in a foot lb rating from the rotation center to the load point.
08-21-2011, 05:43 PM #4
Here is some more information.
Input HP .35
Output 527 LB Inch
Cat No: 718-60-H
The arm shown from center to the end hole is 8 1/2 inches
The cam maximum distance is 5 inches from center.
I guess it is not just lifting the head of the machine but it also moves the squeegee with that 81/2 inch arm. I see we had that welded as well so there must be quite a bit of weight on just that arm let alone the cam that lifts the head.
The bad design I see is the head of this machine is about 50 inches long. The gear reducer is only 6 inches away from the fulcrum point. Lifting the head at the 50 inch point it must weigh about 50 lbs. So at the 6 inch mark on that lever where the cam rides on a bearing there must be hundreds of pounds of force needed just to lift the head.
O------gear---------------------------------------------end of head
Fulcrum 6inches........................................... ..............50 inches
I think I can transfer most of that weight to air cylinders so that would only leave the force needed at the end of that 8 1/2 inches.
So calculating 527 / 8.5 inches the maximum force it should move is about 62 lbs?
If the arm is loaded that much already I may have to design a ball screw drive for that part of the cycle and use just the cam and air cylinders to lift the head or every few years spend another 600 bucks.
I will have to search for a local gear machine shop about the recommendation to replace the brass with hardened steel. I think I can add the air cylinders for a few hundred and that may give me quite a few more years on the brass gear.
I notice the problem is only with about 10 teeth, the rest look fine. I will have to ask the operator where the machine head is at that point of the gear, that may give me a better idea on how to get around that high load point. It may be the weight of the head is putting pressure by pulling the brass gear instead of being pushed by it or the acceleration at that point gives a temporary load exceeding the 527 lbs inch. I think it is the high point of the head where it reverses and comes back down. Relieving that pressure point may be the answer.
08-21-2011, 11:15 PM #5
Don't know a thing about this, but is the area of contact between the gears submerged in lubricant? If the gears aren't making complete revolutions, the contact area needs to be submerged in a suitable lube- with brass a GL-5 gear lube might not be a good choice. Not sure about the additives today, but GL-4 was compatible with brass. No doubt the manufacturer has a specific recommendation.
08-21-2011, 11:35 PM #6
If it's possible to manually actuate the assembly, you could use a beam type torque wrench and get a good idea of the torque involved. Checking the motor current while running would also be a good indicator. A spring scale can also be used to check the torque.
If the reducer in question doesn't make full revolutions, the previous post about lubrication is spot-on. In a case of partial revolutions, you would need to consider filling the gearcase to a higher level.
And it never hurts to check the service manual of the equipment, call the maker of the equipment, etc. and see if they have any guidance regarding the correct gearbox, correct lubricant level, etc.
08-22-2011, 12:09 AM #7
by air cylinders do you mean gas springs? that is what I would consider. Take up most of your weight that way, or maybe a counter weight.
08-22-2011, 06:09 AM #8
Yes the gear unit rotates fully 1 turn per cycle.
Today I did turn it by hand. The most pressure is at the flat spot you see on the cam where it lifts the head from a horizontal position to about 45 degrees. I did verify this is the point of wear, the balance of the cycle is very smooth and takes a fraction of the torque. I did try using a torque wrench on the motor side but the wrench was too big, Am I correct if I did measure 1lb inch on a 60 to 1 ratio it would be 60 pounds? I will get my smaller torque wrench and try it again Tuesday.
I did order 2 gas spring cylinders they have 3 inch stroke and 166 lbs each. That will put the cylinders directly under the gear unit. After I get them installed I may have to make adjustments or add additional hardware, we shall see. I will let you know when I get them installed.
08-22-2011, 07:52 AM #9Yes the gear unit rotates fully 1 turn per cycle.
I would consider filling the gearbox higher than recommended, if any possible overflow would be tolerated.
08-22-2011, 08:23 AM #10
08-22-2011, 01:18 PM #11
Is there any chance of any shock loads on the assembly, or any backlash or chatter as it turns?
Anything that could start to hammer on the worm wheel at that point?
08-24-2011, 10:15 AM #12
The major factor here might be the accelleration and the loads from the rapid starting of the motor. If the motor that drives this is turned off and on by a contactor you might want to look at some sort of soft start, inverters are cheap these days and may be a lot cheaper than upgrading the gearbox or adding counterbalance apparatus.
Look at the cam profile as well, if the lift duration can be lengthened that will help.
A pic of the machine certainly would be helpful too...
08-24-2011, 11:12 AM #13
There may be two things at work here (or more!).
1. The weight factor: If things are too heavy, the strength of the bronze just won't be able to take the load, no matter what the lube is. Can the machine be counterbalanced somehow, like the head of a boring mill?
2. The lube factor: At work we had some worm box failures prior to changing to a Mobile synthetic lube formulated specifically for worm gear boxes. I forget the number, but something like ISO 680 sticks in my mind. Talk to your local Mobile dealer. The boss just happened to price a five gallon pail today, about $184.00 We have experienced no failures since changing to this lube.
08-24-2011, 11:12 AM #14
We retrofitted a screen press that used a similar mechanically linked Screen peel / squeegee motion in a screen printing application.
We decoupled the two motions (which allowed quite a bit of improvement on the peel control and image quality) and it allowed sizing things so they stopped breaking all of the time.
The 8x a minute duty cycle - likely you do a single revolution in about 3-4 seconds to allow for dwell between strokes. That should be fast enough to allow lube to stay on the gear during the cycle. (you also have to realize that the gearbox you are using is a bit of a piece of junk compared to a quality box by someone like Cone Drive or David Browne)
My hunch is that the force to pull the squeegee at mid stroke + the peel lift force is exceeding the torque capacity of the gearbox.
Since I don't know the details of the mechanism - I assume that the lever arm is the squeegee link indicating that you have a 17" stroke?
The units that we made allowed the operator to crank up the tension on the squeegee and adjust the rate of peel as a function of squeegee position. This screen press made prints that were much larger and the forces a lot higher - but all the same, we were able to crank the speed up quite a bit - a lot faster than 8 prints a minute by a long shot.
Last edited by motion guru; 08-24-2011 at 03:18 PM.
08-24-2011, 08:14 PM #15
Yes, the arm is for the squeegee. This is our oldest small press that really doesn't get used much compared to the other presses. It is the only American made press we still have operating. All the others are German and Swiss made. Don't know why but you just can't buy quality machinery in the U.S.A. anymore. Well John Deer tractors is an exception.
I will have the gas spring cylinders today, I will post a pic when I get them installed.
You are correct on the force is greatest when the head is lifted, the motor is a DC motor, I guess it could be slowed down on the initial motor start, I will see if I can ramp it like you would a stepper motor. That would be a good thing to do.
I too, thought of separating the squeegee drive. That may be another option. Once I get the press running again i will see what the forces are with a squeegee installed with pressure. The head weight may be an issue if I make it too light since the squeegee rely's on the head weight. My thought is to use a solenoid to lock the head down during the squeegee stroke. I would make that using heavy rubber and perhaps springs so it would only apply pressure to the head but could break away if the solenoid did not open because of an electrical fault. I guess a sensor to sense the solenoid position, a micro switch N.O. when head is locked could be closed when the lock is open could shut down the power in that event.
09-16-2011, 02:13 AM #16
BOSTON 51949 F718-60-B5-H CENTER DISTANCE: 1.8 INCH RATIO: 60:1 INPUT FLANGE: 56C OUTPUT SHAFT: 0.875 INCH PROJECTION: DOUBLE MAX. INPUT RPM: 3600
MRO # 42650
Your Price (ea) $376.01*
Brand BOSTON GEAR
09-16-2011, 10:01 AM #17
Only three posts by mrosupply, and EVERY single one is Only spamming his site.....
Is this a pattern?
09-16-2011, 04:02 PM #18
Also are also available for technical assistance and product sourcing assistance. =)
09-16-2011, 05:02 PM #19
09-16-2011, 05:24 PM #20