Force to open the buckle of a ratchet

[jai_helsing]

Hi,
I have strap ratchet which is being pulled on both sides with some force(say 500lb). Could some one please help me know the force to be applied on the handle of the buckle to open it? Can anyone please help me with the math in this problem. The ratchet is being pulled by webbing straps on both sides.

 

[sophiecentaur]

Quick answer is that the load on the strap times the radius of the roller equals the Force applied times the length of the 'lever' / handle. You would need to decide precisely where you can consider the force from your hand to act. The ratio of the forces is the reciprocal of the ratio of the distances;
If the diagram is to scale then a rough estimate of distances in the diagram gives the roller diameter equal to say 1.8cm and the radius 0.9cm. The length of the lever (from centre of roller to the red F line is about 8cm - call that 9cm so the applied force will be 1/10 of the load on the strap i.e. 50lb. If you want it more accurate than that, measure the actual device. That figure of 1/10 could be optimistic because it doesn't take into account friction in the bearing so the actual 'mechanical advantage' would be somewhat less than 10.
Note: the force on the strap is F and not 2F, in case that was worrying you.

 

[CWatters]

,+1

The tension in the webbing create a torque on the roller/drum that forces the ratchet against the pawl. In order to release the pawl you have to use the handle to relieve most or all of this torque so you can overcome the friction and pawl spring.

If you hit the pawl with a hammer it might be possible to open it without applying any torque to the handel but that's not recommended.

 

[sophiecentaur]

I omitted to mention that you need to relieve the tension in order to pull the pawl out of the ratchet slot.

 

[anorlunda]

Hmm, that is a classic ratched and pawl. Your question is the force required to lift the pawl. That will depend on the angle of the ratchet teeth, and the friction between pawl and ratchet. I'm going to move this to Mechancical Engineering. Hopefully, one of the MEs can help.

 

[sophiecentaur]

Your question is the force required to lift the pawl.

I'm not too sure about that. The pawl will be held in with a spring that's just sufficient to pull it into the ratchet teeth and that won't require much force from the thumb(?) as your hand is pulling up the lever handle. That force can't be known from the picture supplied but it will have to be enough to make the pawl drop into the slot reliably and give the user reassuring clicks as it runs over the ratchet teeth when tensioning the strap.

 

[jai_helsing]

Thank you all for your valuable suggestions to solve this problem. This solves part one of my problem. The part 2 just as specified by anorlunda would be to figure out the force required to lift the pawl. That is the force being exerted by the pawl on the gear wheel teeth. How to calculate the force between the pawl and gear when say 50lb is being applied on the handle. Is it going to be the same?? Thank you in advance.

 

[anorlunda]

I use those ratchet straps too, and I know how difficult they can be to release because of high forces needed to release the pawl and the awkward angles. I use a tool like the shaft of a screwdriver to help release the pawl. That makes it abundantly clear that the force to lift the pawl is proportional to the tension force on the straps. Many winches have an external lever that can be used to rotate the roller a bit more, leaving zero force on the pawl temporarily so that you can lift it easily. The tiedown straps in your picture, don't have that feature.

If the pawl had a point, then it a matter of leverage. The strap force times the radius of the roller must balance the pawl force times the radius where the pawl touches ratchet. However, the face of the pawl is not a point, it spreads over a significant fraction of the ratchet radius, making the calculation difficult.

Compounding that is the angle of the ratchet faces. If exactly radial, then only friction forces oppose pawl motion. If >90 degrees, then lifting the pawl requires stretching the strap even more. If <90 degrees then lifting the pawl reduces strap tension; but that is unstable and might result in spontaneous release. If the ratchet face has a curve, it is more difficult still.

Another factor that @sophiecentaur pointed out is the leverage in the mechanism used to lift the pawl. I do not think the pawl spring force is significant in this problem.

I was hoping that the M.E.s here have specifically studied ratchet-pawl winches and could give us the math they used.

 

[sophiecentaur]

spontaneous release.

There are many slightly different mechanism to perform this function
In the types of strap / ratchet systems I have used, there has not always been a very 'controlled' way of releasing the strap tension as I remember it. Part of the mechanism which hasn't been discussed here involves the drum spindle being able to move in slots and tension due to the lever or pawl holds the barrel against a surface, gripping the strap. When the lever is raised to take the stress off the pawl it 'takes over' the tension and can release the tension when it's lowered. The strap then slips through the slot unless you are really careful about keeping the force on the handle. There are similar brake mechanisms used in abseiling and they are better designed and more expensive (not surprisingly) because they have to manage many metres of rope going through and the ability to regulate speed - unlike the versions used for tying down loads.

 

[Gracie G Sheppard]

Quick answer is that the load on the strap times the radius of the roller equals the Force applied times the length of the 'lever' / handle. You would need to decide precisely where you can consider the force from your hand to act. The ratio of the forces is the reciprocal of the ratio of the distances;
If the diagram is to scale then a rough estimate of distances in the diagram gives the roller diameter equal to say 1.8cm and the radius 0.9cm. The length of the lever (from centre of roller to the red F line is about 8cm - call that 9cm so the applied force will be 1/10 of the load on the strap i.e. 50lb. If you want it more accurate than that, measure the actual device. That figure of 1/10 could be optimistic because it doesn't take into account friction in the bearing so the actual 'mechanical advantage' would be somewhat less than 10.
Note: the force on the strap is F and not 2F, in case that was worrying you.

The crank arm ratchet and ratchet lock can rotate flexibly. When the lock is in the locked state, it needs a large external force to open, and the lock itself has good strength and rigidity, and can withstand a large external impact.

 

[Baluncore]

There are two spring loaded pawls. One is mounted on the handle and one on the base plate. The webbing is tensioned by repeatedly moving the handle backwards and forwards.
The radius of the roll of webbing increases as more webbing is wound onto the roller, until it pushes the base ratchet back, preventing further webbing being drawn onto the roll.

To release the webbing, the handle pawl is pulled back against a spring to be clear of the ratchet wheel, so the handle can then be opened fully to 180°. An eccentric cam part of the handle then actively lifts the base pawl from the ratchet wheels which suddenly releases the tension in the webbing.

The release force is difficult to estimate because the webbing tension is released by an unspecified cam, lifting the pawl, with possibly lubricated friction against the ratchet wheel, and an unknown radius spool of webbing. Suffice it to say, that the force on the handle needed to tension the webbing is usually greater than the force needed to release the tension.

 

[anorlunda]

To release the webbing, the handle pawl is pulled back against a spring to be clear of the ratchet wheel, so the handle can then be opened fully to 180°. An eccentric cam part of the handle then actively lifts the base pawl from the ratchet wheels which suddenly releases the tension in the webbing.

I used those things for more than a year, trying to lift the pawl under load with great difficulty. Even with the help of a screwdriver, it was still difficult. Eventually, I saw a Youtube video about the cam action that @Baluncore described, and found that made the release almost effortless. As the Car Talk guys would say, "Dope Slap."