Next generation snatch block

[DaveInDenver]

Maybe someone more versed can explain but seems like you can see from the capstan equation that field test makes sense.

Capstan Equation:




Assuming the lowest possible coefficient of friction for UHMWPE of μ=0.05 (0.05 to 0.07 is what Dyneema specifies μ is for yarn-to-yarn) the resulting load force from bending the rope 180° (e.g π radians) around the ring will be increased to 117%. Assuming the low range of μ=0.10 that has been cited the capstan formula indicates 137% resultant force and the high μ=0.14 yields 155% increase in load force.

That he saw 169% higher compared to a sheave with a bearing to deal with the rotation seems to indicate it might be worse than theory predicts. Isn't this exactly why bearings were invented in the first place?

 

[Hillbilly Heaven]

So why don't we put a sealed bearing inside the aluminum ring? Make it wider so the sides of the pulley don't rub the soft shackle. I don't think it is speed that is harmful. We aren't catapult launching aircraft. I do think bend radius are an issue. If rings are same diameter as the sheave in the snatch block then that is not a problem. Maybe the bend radius of the soft shackle is too small requiring a thicker ring.

 

[Metcalf]

Maybe someone more versed can explain but seems like you can see from the capstan equation that field test makes sense.

Capstan Equation:

View attachment 558729


Assuming the lowest possible coefficient of friction for UHMWPE of μ=0.05 (0.05 to 0.07 is what Dyneema specifies μ is for yarn-to-yarn) the resulting load force from bending the rope 180° (e.g π radians) around the ring will be increased to 117%. Assuming the low range of μ=0.10 that has been cited the capstan formula indicates 137% resultant force and the high μ=0.14 yields 155% increase in load force.

That he saw 169% higher compared to a sheave with a bearing to deal with the rotation seems to indicate it might be worse than theory predicts. Isn't this exactly why bearings were invented in the first place?

Why was just running the same UHMWPE line around the shackle 180 degrees even less load than the traditional snatch block?

 

[Metcalf]

Maybe someone more versed can explain but seems like you can see from the capstan equation that field test makes sense.

The capstan equation is a hold-force study, not a bearing drag study.

 

[Metcalf]

So why don't we put a sealed bearing inside the aluminum ring? Make it wider so the sides of the pulley don't rub the soft shackle. I don't think it is speed that is harmful. We aren't catapult launching aircraft. I do think bend radius are an issue. If rings are same diameter as the sheave in the snatch block then that is not a problem. Maybe the bend radius of the soft shackle is too small requiring a thicker ring.

The 'safe' non-moving ( linear) bend radius for Dyneema is about 2-3x diameter to reach maximum MBS

 

[DaveInDenver]

So why don't we put a sealed bearing inside the aluminum ring? Make it wider so the sides of the pulley don't rub the soft shackle. I don't think it is speed that is harmful. We aren't catapult launching aircraft. I do think bend radius are an issue. If rings are same diameter as the sheave in the snatch block then that is not a problem. Maybe the bend radius of the soft shackle is too small requiring a thicker ring.

That's essentially what I'm wondering.

 

[Metcalf]

.....I wonder if most snatch blocks on the market use a bushing or a bearing?

 

[DaveInDenver]

The capstan equation is a hold-force study, not a bearing drag study.

Can you help with references to understand how to analyze this? It seemed like finding the force of the ring following the winch rope around would be determined by the belt friction (e.g. capstan) equation and that would be equivalent to the force dragging the ring through the soft shackle since there isn't a bearing or traditional bushing translating motion.

 

[Metcalf]

Can you help with references to understand how to analyze this? It seemed like finding the force of the ring following the winch rope around would be determined by the belt friction (e.g. capstan) equation and that would be equivalent to the force dragging the ring through the soft shackle since there isn't a bearing or traditional bushing translating motion.

The capstan equation would only give you the maximum force ( which would have to be converted to some kind of angular moment ) of what the relationship between the input force and coefficient of friction is. It could be said that the angular force ( ring spinning ) is always going to be LESS then the bushing drag ( because the ring is still turning ). With the right setup you might be able to tell where that crossover point load is using the capstan equation. Basically, at what point does the winch line tension start turning the ring vs sliding around it or vise-versa.

Another way to think about this with the capstan equation. If you wrapped the winch line 540 degrees around the ring, the capstan equation would spit out a HUGE force number because you increased angular contact length, but the bearing force isn't going to be any greater on the soft shackle. Now, if you could find JUST the right wrap angle of line where the ring JUST started turning at a known force, you could surmise that might be close to the total bushing drag coming from the soft shackle to ring interface.

If the ring WASN'T turning using the capstan equation, we could know that the bearing drag was a greater number ( minus some leverage from the different diameters )
Since the ring turns we can't really say that.

There are probably some equations for spherical bearing load that could be used. I think the way the soft shackle is typically loaded is similar ( but only in 2d basically). I noticed in my testing that it was best to keep the legs of the soft shackle separated to the width of the ring to prevent extra drag( and risk of abrasion ) on the smaller radius on the OD of the ring. Basically, if whatever you are hooking the 'other' end of the soft shackle to has a smaller radius than the width of the ring you will be 'pinching' the sides of the ring causing extra drag. I never really wanted to quantify what that was, but with my anti-tip block device it seemed like a good call to design that geometry in.

 

[gungriffin]

Pull out your wallet..
Adding sealed bearing will increase manufacturing time and parts count considerably. Probably weight increase also.

It seems like at that point what you would have as well is just a fancy looking snatch block with nothing easily helping to holding the rope in place....

It is an interesting test. Having data is good. I am just not seeing the same thing happen in my testing. I'd love to have a load cell to test this stuff out in a similar manor. I do have some significant differences in how I am setting up and running my snatch rings.

The results of the video were much different than I expected to see. I actually thought that the pulley might win before watching. I don't get where the difference comes in. I also am not an engineer and don't understand all these fancy equations that have been posted. The video seems legitimate to me and was produced by someone without a horse in the race.They showed the real world difference based on what seems to be a good experiment. Now it just remains to see where this tech goes as it improves. I look forward to reading/watching about more testing of these pulleys. Maybe there is a possibility to put some sort of low friction coating on the pulley? Sort of like what they are using in the cylinders and pistons of high performance engines now?

PS I am definitely not trying to poo poo on these pulleys. To be clear, I want to see evidence to the contrary of the video. I want to love them. This is mainly because I love simple and elegant solutions to problems. In time, I believe the tech will improve.

 

[Metcalf]

It seems like at that point what you would have as well is just a fancy looking snatch block with nothing easily helping to holding the rope in place....



The results of the video were much different than I expected to see. I actually thought that the pulley might win before watching. I don't get where the difference comes in. I also am not an engineer and don't understand all these fancy equations that have been posted. The video seems legitimate to me and was produced by someone without a horse in the race.They showed the real world difference based on what seems to be a good experiment. Now it just remains to see where this tech goes as it improves. I look forward to reading/watching about more testing of these pulleys. Maybe there is a possibility to put some sort of low friction coating on the pulley? Sort of like what they are using in the cylinders and pistons of high performance engines now?

PS I am definitely not trying to poo poo on these pulleys. To be clear, I want to see evidence to the contrary of the video. I want to love them. This is mainly because I love simple and elegant solutions to problems. In time, I believe the tech will improve.

The kicker in that testing video is that just wrapping the line around a shackle was even more 'efficient' than the traditional snatch block.

UHMWPE is already a really good low friction, basically self lubricating, material against a smooth hard surface like the aluminum of the snatch ring.
You can commonly buy UHMWPE bushings off the shelf that are commonly used in many industries for similar style loading.

It's a weird test to me so far. I have watched it a few times.
I am just not seeing anything similar in all my testing done. I guess I need to step up my game with a load cell and see what I can figure out......

 

[gungriffin]

The kicker in that testing video is that just wrapping the line around a shackle was even more 'efficient' than the traditional snatch block.

UHMWPE is already a really good low friction, basically self lubricating, material against a smooth hard surface like the aluminum of the snatch ring.
You can commonly buy UHMWPE bushings off the shelf that are commonly used in many industries for similar style loading.

It's a weird test to me so far. I have watched it a few times.
I am just not seeing anything similar in all my testing done. I guess I need to step up my game with a load cell and see what I can figure out......

Any chance that a local crane supply place might let you borrow a load cell to take some footage?

It also seems like it might be interesting to reach out to the Youtube guy to inquire about how he performed the test. I really look forward to seeing more testing. Using the shackle to redirect the line surprised me. Especially how efficient he said it was, but then went on to say that it created a large amount of heat.

 

[Metcalf]

Any chance that a local crane supply place might let you borrow a load cell to take some footage?

It also seems like it might be interesting to reach out to the Youtube guy to inquire about how he performed the test. I really look forward to seeing more testing. Using the shackle to redirect the line surprised me. Especially how efficient he said it was, but then went on to say that it created a large amount of heat.

I have some access to load cell stuff at work....but everything is basically on either end of the spectrum we need. I'm not going to use a 50K load cell for testing....and my little 2000lb hanging scale could get damaged.

Yeah, bending a moving line around that kind of radius is going to be hard on it. That violates some 'rules' for sure.
It is nice to know that it could be done in a pinch if needed though.

I think another interesting comparison would be to run the working end of the line through the center of the hole vs around the ring. This would be similar to the 'shackle' test but with a larger radius that would be smoother. This is how a lot of these low friction rings are used in the arborist industry.

 

[Charles R]

I would like to see him repeat his testing, but on a nice even concrete slope, and with two lead cells. I saw this video awhile back, and the numbers didn't jive for me either. (I'm the commenter XPGhia) Personally I think there's a total load variable, like a rock partially inhibiting the movement, that's being incorrectly attributed to fiction.

One thing that didn't make sense to me, and maybe someone else can explain it so i get it, is the increase in load measured in his setup.

To me...

Where he set the cell, I would be expecting a decrease in load as friction increased, and it could never be higher than 50% of the total load.

To view it another way... If we somehow get zero slip (100% friction?) from the ring to the shackle, we get a total bind. No rotation of the ring. Then, let's add zero slip to the winch line. What happens? In my mind, the line on the winch side of the pulley now sees 100% of the load, and the other 'anchor' side goes full slack.

Basically, since a full bind of the pulley would lock up the rope at the pulley, you end up with nothing but a single line pull.

Coming back to his testing. I can't see how the load cell should ever see an increase at all. To me, if there was a perfect zero friction setup, the most that cell would see is 50% of the total load.

 

[gungriffin]

I would like to see him repeat his testing, but on a nice even concrete slope, and with two lead cells. I saw this video awhile back, and the numbers didn't jive for me either. (I'm the commenter XPGhia) Personally I think there's a total load variable, like a rock partially inhibiting the movement, that's being incorrectly attributed to fiction.

One thing that didn't make sense to me, and maybe someone else can explain it so i get it, is the increase in load measured in his setup.

To me...

Where he set the cell, I would be expecting a decrease in load as friction increased, and it could never be higher than 50% of the total load.

To view it another way... If we somehow get zero slip (100% friction?) from the ring to the shackle, we get a total bind. No rotation of the ring. Then, let's add zero slip to the winch line. What happens? In my mind, the line on the winch side of the pulley now sees 100% of the load, and the other 'anchor' side goes full slack.

Basically, since a full bind of the pulley would lock up the rope at the pulley, you end up with nothing but a single line pull.

Coming back to his testing. I can't see how the load cell should ever see an increase at all. To me, if there was a perfect zero friction setup, the most that cell would see is 50% of the total load.

You can see that the load becomes fairly consistent with the point that he chooses to measure it at. I don't see how that would be possible if there were rocks and stuff in the way.

The increase in friction increases the weight of the pull because the friction causes it to pull more at the hitch mount. Probably not saying that with the correct terms, but someone smarter will chime in.