Ratchet Brake

[jdkuhndog]

I am trying to size a spring which will engage a ratchet into a ratchet wheel. There is a solenoid which diengages the ratchet (by overcoming the spring force) and then the spring will re-engage the ratchet when electricity is shut off.

Is there a 'rule of thumb' or Standard which suggests how much force the spring should inflect upon the ratchet in order for engagement? This force will have nothing to do with holding the brake...the profile of the ratchet will take care of that...this is unusual I guess because gravity won't be acting on the linkage...it is coming up from underneath the ratchet wheel.

Any help would be appreciated.

Thanks.

 

[c2s]

In lieu of any standards, I would think that solenoid sizing would be the primary design consideration ($, packaging, throw, sticktion, etc) and determine the spring required.
Kevin

 

[jdkuhndog]

solenoid sizing is really not an issue...that is easy to figure out how much force is required to open up the ratchet...I just need to know how much pressure the spring should put on the ratchet while closing the pawl. 1 pound of normal load between the pawl and the ratchet wheel seems like it would be too little (not enough safety factor in there to ensure it would close everytime). Would 5 pound of force be too much ?? The ratchet wheel is 15" diameter and the surface area of the pawl against the surface of the ratchet wheel is about 1.2 in^2...the surface of the pawl against the tooth (not important for this discussion) is about 1 in^2.

Thanks for any help.

 

[Barry1961]

I think that c2s has a point that the solenoid may end up being the defining factor. How much spring force can your solenoid overcome and hold? This over course is added to the force to dis-engage the pawl.
The required force to engage the pawl will depend of the bearing the pawl pivots on and the conditions it is operating in. Is this operating operating outside in a mill enviroment or enclosed inside a oil filled box? In the past I have found that having a cheap fish scale handy as a force indicator often was the best way to wag a required force.

Barry

 

[israelkk]

The pawl-ratchet wheel arrangement should be designed such that when the ratchet wheel is pressed against the pawl the resultant force pushes the pawl toward the center of the wheel and not outward. Therefore, the spring force never intend to resist against the force between the ratchet wheel and the pawl but only to make sure that the pawl will move to the next tooth when the ratchet wheel move away from the pawl.

Therefore, the solenoid should be able to overcome the maximum friction force between the ratchet weel and the pawl plus the spring force.

 

[jdkuhndog]

The solenoid is sized for 40 pound force. With the lever advantage it has over the spring, there is really no concern for this.

We have a special design that requires the hoist to be raised prior to the solenoid acting to release the brake...this will take friction between the pawl and ratchet wheel out of the equation. It would take me an hour to type out the reaons why we are doing it this way...but trust me that it is required.

The application is inside in controlled conditions.

I guess my concern was, I want to select a spring that will give enough force to the pawl but would not be compressed beyond its recommended max. compression when the pawl is opened (when the solenoid is acting). The spring the desinger selected only barely overcame the weight of the pawl unless its initial compression was at a point that when the solenoid opened the spring would be 'overcompressed'. The spring I have selected would put a 5 pound force on the pawl in the direction of the wheel...I just didn't want this to be too much of a force so that it would cause a loud bang everytime the solenoid released.