Break Stopping Times

First of all, you have to calculate the assembly momentum of inertia. With this and the speed, you can calculate the total energy associated with the movement.

Then, you need to know what is the force that your brake is able to apply to the rotor. For this one, you need to know with reasonable accuracy the dynamic atrict force during actuation.

With this values, just apply the equation for angular acceleration to discover how long it will take to stop the assembly.

Also with the energy associated, you can calculate the heat input to the brake and take it in account in case of repetitive or cycling actuation.

I'm affraid this assembly will not just stop immediately as mentioned...

I once watched a demo video regarding a safety device for saws in the size similar to the one you are mentioning. This device detected the small variation in conductivity (or capacitance, I'm not sure...) that occurred when a human finger touches the saw, and trigger the safety shutting down all the stuff and avoiding the finger to be cut off, but only lightly scratched. A good idea. There was a demonstration using hot dogs... But, for your reference, the safety device included an aluminum block that was inserted in front of the saw teeth to make it stop (by just crunching the teeth in the block). It took like 4 teeth at least to stop the saw.

I was well aware of the "Saw stop" device. I was tossing around the idea of doing the same thing using a prony brake type device that would stop the blade almost instantly. It would be a huge advantage over the "Saw stop" device in that the blade would not be damaged and there would not be any device to replace after a trip. I guess the idea is now out of the bag for any tool company to develop.

I'm sure many people had that same idea for many years. The regular braking concepts just don't work fast enough or we would have seen a fast safety brake 40-50 years ago. The only way to prevent injury is to retract and stop the blade within a couple mm of circumferential rotation. The saw-stop device may seem excessive, but that is why it works.

Let us assume the inertia of your rotating system be J (kgm)^2 and the brake torque as constant Tb Nm then

the equation is deceleration = ε = Tb/J (rad/s^2). This is the rate with which the speed will be decreased. The speed at time "t" from brake application is ω (rad/s)= ω(0)-ε*t(s). This gives the time to stop as t*= ω(0)/ε.

The angle is ∫ω*dt= ω(0)xt-εxt^2/2. The angle being in radian the a(°) = a(rad)*180/pi

Angle to stop is a*= ω(0)xt*-ε x t*^2/2 =ω(0)^2/(2xε)

Now the travel will be R(m)*a*(rad).

If you introduce the values then you get the answer.

As you see the time depends on the accumulated energy and the brake torque applied.

Hope it will help

I'm afraid the math is a bit beyond my capability, but thanks anyway.

Being scared of mathematics, it means you have an analogous value faculty. I also have the same problem of analogous memory and run away from threatening mathematical signs.

How ever assigning digital values and behavioral patterns calls for mathematical expressions. The best part you can do is to go for a very simplified parameter behaviour expression. I appreciate your frankness in admitting.

Best wishes for your instant brake research.

Regards

http://www.youtube.com/watch?v=LGiYlyo2-eQ&feature=related

this video is of a braking device for a power saw, they put a small current through the blade. When the current is broken, by touching skin, human or hot dog, the blade stops and retracts.

this is about as fast as I've see anything stop.

Laby

I can recall a long time ago when a car was up on jack stands and you spun a wheel and then applied the brake; the wheel being around 27" dia with a lot of inertia, it would stop instantly. The Saw stop doesn't stop instantly, but fast enough to prevent injury. It's stopping speed is in the millisecond range and does leave a small nick. (certainly better than taking the whole finger) A disc brake setup might also work.

ronseto... No insult intended, but the appearance of a car wheel stopping instantly is insufficient proof of concept. Some actual measurements need to be taken. The saw-stop decelerates the blade very quickly to prevent significant injury. You appear to understand its operation fairly well, but here are some numbers for consideration:

10 [inch] saw blade @ 5000 [RPM] = 66.5 [m/s] circumferential velocity
IF the stopping distance is 10 [mm], that is a little over 90,000 [G's] deceleration in about 0.15 [ms].

The result is a mangled aluminum "brake pad" and a damaged saw blade.

Rotational inertia and kinetic energy are important when you consider all the various material properties and where you can safely dissipate this energy. Don't forget to include the kinetic energy in the motor/gears/pulleys as these may be significant.

If none of the numbers above seem beyond your design concept, keep at it! Otherwise, file the current idea away for possible use elsewhere. I usually keep dated "idea" notebooks and suggest you do the same (if you don't already). Even if your current idea doesn't quite fit this application, it may have merit in other areas. Best wishes.