Hi all!
We have an arm that’s attached to 2 chains. I noticed before we turned on the motors that the chain tension is different at different spots.
If you look at the photos, you’ll see that the top of the chain is completely loose while the bottom is not. When the arm is in resting position, both the top and bottom are well tensioned.
I don’t know if this is normal or not, so any help would be greatly appreciated. Thanks!
It is possible that one of your sprockets or axles is eccentric. This would cause tension to vary with rotational position of the assembly.
There is backlash in the system that makes one end appear tight while the other is not because the weight of the arm “falsely” pulls one end tight. In reality, the entire chain loop is not tensioned and the tensioner shown in the images needs to be tightened.
Don’t you just turn the turn buckle until the chain is tensioned? Or is the chain tension changing through the range of motion?
Thanks for this
When the arm is in the closed position, and there is no real weight on the chain, the chain is fairly tight. But, when the arm is extended outwards and weight is on the chain then we have the looseness issue.
Do you think I should tighten the turnbuckle when the arm is fully extended ?
You should see some slight change in chain tension depending on the state (Loaded vs. Unloaded) of the arm. Basically, the side “Doing the work” (Being Pulled) should get tight, while the other side will get loose. Hard to quantify without knowing specifics, but anything that is well under 1/2 of a link or so is likely a non-issue. (So anything less than .0625 from the centerline of the chain (.0625x2=.125, which is about half of the pitch))
One other thing to look out for is the system flexing under load, or as mentioned above, the Sprocket not being Concentrically Mounted. Either of these things will cause dynamic changes in tension. Easy way to tell one or the other is to watch the Drive Shaft of the Arm as you apply load, if you see it bend with the naked eye, it’s likely enough to cause what you’re seeing. Adding tension will actually make things worse in the long run. (It may tighten the chain, but you’re also pre-loading the shaft, which is less than ideal.)
As far as Concentricity goes, While in the “Tightest” Position - You can mark the sprocket at the “Top” (Draw a line between the axles, then mark the tooth aligned with this line, on the opposite side of the Chain Run (Looks like the “Top of the Arm” in this case) Remove the chain, and then measure the distance between the tooth and center bore at this point, then at some number of other points on the assembly. They should all be approximately identical (within a few thou if not better) - if you’re out, you should know right away.
Also looks like your drive sprocket has some anodize wear on it. How much use has it seen? A worn sprocket can cause some of what you’re seeing, especially if the wear is asymmetric.
Wow thanks so much for the info!! The bottom sprocket is used, but the top sprocket is brand new. We haven’t actually run this system under motor control.
Thanks for the info. Tomorrow I’ll put the chain under tension to see if the drive shaft is bending at all. If not, I’ll mark the sprocket and see if it’s bent inward at all.
I should’ve mentioned this; but on the other side (closest to the camera) there is another chain in the exact configuration. It is having the same type of tension issue. My guess is that the shaft may be bending under the weight, but yet again, it’s only a 28” piece of pvc pipe. Not that much weight.
Thanks for the info!
Generally, if the arm is always going to be loaded by gravity in only one direction (i.e. you don’t flip the arm over to the other side), then you will always have one side of the chain loop in tension and the other side will be the slack side. I’m a little confused by your pictures in that it seems like in the first 3, the arm in question is out of the frame of the picture, but I am assuming that it is off to the right side of the picture and that gravity is acting downward such that that would put to top of the chain loop in tension and the bottom would be slack. But you seem to be showing the top run of the chain is slack. Is the arm resting on the ground or being pulled hard against the ground in those pictures?
In any case, the slack may be the result of closing gaps in the system as all the components are loaded. The bearings have small clearances between the bore and the shaft. There are small clearances between the shafts and the sprockets, there may be clearance between the structure and the bearing. All those clearances can add up when everything is loaded to enough shifting of centerlines to cause slack in the chain.
The slack should not be a problem. You could add a spring loaded idler sprocket to the unloaded side if it is bothering you or to dampen out any bouncing that the slack might be causing in the arm position.
Thats the weird part. Im posting a video now, but if you look the chain closest to the camera has tension on the top while no tension on the bottom. The chain farthest away has tension on the bottom with no tension on the bottom.
Its connected to an arm (a piece of PVC that is 28" long) and is not resting on the floor. The gear reduction and motor are keeping it up off the floor.
[quote=“wgorgen, post:8, topic:426139, full:true”]
Generally, if the arm is always going to be loaded by gravity in only one direction (i.e. you don’t flip the arm over to the other side), then you will always have one side of the chain loop in tension and the other side will be the slack side. I’m a little confused by your pictures in that it seems like in the first 3, the arm in question is out of the frame of the picture, but I am assuming that it is off to the right side of the picture and that gravity is acting downward such that that would put to top of the chain loop in tension and the bottom would be slack. But you seem to be showing the top run of the chain is slack. Is the arm resting on the ground or being pulled hard against the ground in those pictures?
In any case, the slack may be the result of closing gaps in the system as all the components are loaded. The bearings have small clearances between the bore and the shaft. There are small clearances between the shafts and the sprockets, there may be clearance between the structure and the bearing. All those clearances can add up when everything is loaded to enough shifting of centerlines to cause slack in the chain.
The slack should not be a problem. You could add a spring loaded idler sprocket to the unloaded side if it is bothering you or to dampen out any bouncing that the slack might be causing in the arm position.
This would indicate that the chains are fighting each other. This would normally mean that the indexing of the sprockets was not synchronized. I’m not sure what sprockets you are using, but most have timing marks on them to help with alignment. If the sprocket tooth count is not a multiple of 6 then rotating them relative to the shaft by 1/6 of the rotation will cause the teeth to be in a different relative position to the other sprockets and result in a partial chain link difference between the two chain loops.
The bottom sprocket is a 16tooth and the top is a 44 tooth.
How do I fix fighting chains then ?
Thanks for the info by the way, super helpful!!!
You need to index the sprockets the same way on both sides. If there are timing marks on the sprockets you want to line up the timing marks of the sprockets that are on common shafts to be pointed in the same direction relative to the shaft. If there are no timing marks on the sprockets, the best thing to do is lay them on top of each other and then rotate them until both the shaft features (hex or bolt pattern) and the tooth pattern line up. Then mark them and install them on the shafts in the same orientation.
Super helpful advice. Thank you so much!!! I’ll get to work tomorrow and see if that fixes it.
John
It is almost definitely a timing issue. You have 2 locations that could be causing it.
The 16 tooth is not divisible by 6 or by 3, but is divisible by 2. This means that 2 of the 6 possible orientations can be used properly to line the teeth up.
The 44 tooth is not divisible by 6 or 3, but is divisible by 2 as well. So again it will have 2 options for alignment.
If you were using a 42 and an 18 tooth you would be able to use ANY of the 6 possible positions.
1678 has in the past made our own chain sprockets for this exact kind of set up and we broached the hex bore with 2 sprockets at the same time to make sure they could be properly timed.
5458 has done similar broaching where the teeth could be lined up exactly 1/2 of a tooth off so you could get two chains closer together on the same shaft.
The chain and tensioners are dragging over the boxtube which significantly messed with the tension. Larger sprockets or a different crossbar should be used to avoid excessive friction and changing tension due the tensioners raising on the boxtube.
