Sprocket ?

chawalang

Well-Known Member
I wanted to get peoples thoughts on a design decision I found on a winch for a fire door. Looking at the motor there is the sprocket on the shaft on the gear box that goes to a shaft on two pillow blocks. It goes to a sprocket that is a little smaller, there is another sprocket on the shaft that is even smaller that then goes to the sprocket on the drum. Can someone please explain to me this design decision. I have a few thoughts on it but wanted some clarity. I know how to do the math going from motor, motor box, speed reducer sprocket then to drum sprocket to determine the load it can max hold minus 25%, at the max speed but was curious about this. this obviously looks like in has a few more steps.



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A couple of guesses--and nothing more than guesses, not even very educated ones:

Is it something as prosaic as the motor (or gearbox) came with a sprocket for a chain of pitch X and the drum one for pitch Y? It's hard to tell from the picture, but they do kind of look different.

The gearbox shaft may be too big around to permit the mounting of a sufficiently small sprocket to get the right ratio if chained directly to the drum. I guess if that were the case one could take the output shaft out and machine it down on a lathe, but perhaps putting in a countershaft was quicker or easier or cheaper.
 
You are sure the shaft on the pillow blocks doesn't do anything? It doesn't pass into the housing? No governing nor braking function or over-speed brake?
 
The shaft on the pillow blocks just has the two sprockets on it. There was no extra safety device attached to it either, it just sat independently.

Looking at the photos again the shaft on the pillow blacks does look smaller than the shaft coming out of the gearbox.
 
The motors for our electrics have a pretty similar set up except the chain right out of the motor is even smaller, leading to the sprocket that drives the lift chain. That being said there is about 70 feet from the headblock to the motor, and the lift chain goes into that block so the second sprocket isn't really driving like in your scenario.
 
It may not be related to speed/torque but rather the physical path of the chain. The intermediate sprockets might be to keep the chain from rubbing against framing or other objects.

Just a guess.

Ethan
 
It might also be a step to protect against side loading the shaft coming out of the reduction box. There is already a pretty beefy gusset around the split bearing on the end of the primary drive gear. Perhaps there was a failure in the past due to excess side load and the pillow block/reduction set up was installed to lower the gear ratio even more AND act as a buffer between the load and drive sides.
 
As I look at the photos further, this all looks a little homemade. Hardly manufactured system with testing and listings. The lack of guards is one dead give away. It may have been a case of what was on hand or readily available.
 
Odd piece - is this basically a traction drive head block on a counterweight door? Drum made me think roll up door.

In the second fuzzy photo - what is the wire rope attached to the piece of steel extending from the gear box shaft bearing?

How does this close in a fire? I'm missing a clutch orsome release to disconnect the gearbox to allow the door to close on its own without power, the essence of a fire door.

All very odd and for me at least incomplete.
 
I've seen some machines that do similar as a mechanism to allow one to disengage the motor for repair without having to remove or disengage the driven element. You just loosen the bolts on the motor, tilt it forward, and pop the chain out but you've still got everything else mechanically coupled. That said, looking at how this is arranged, I think @egilson1 has the best answer here - chain pathing, along with possibly some mechanical or available-parts advantage. The motor shaft has to be supported where the chain to the drum has to be.
 
Another random thought: I wonder if initially there was a handle attached to the countershaft that one had to turn eighteen gazillion times to move the curtain, and at some point an enterprising individual got tired of doing that and cobbled up the motor arrangement.
 
This is a fire safety curtain - not a door - that confused me. It's on a grid. Thats a clutch on the gear box and releasing the cutline (seen in second photo) allows that bar to pivot up stage, releasing clutch, and curtain falls/closes. This looks pre-governor age, so maybe there is a piston under counterweight to slow it or maybe the bulky piece on head block governs speed - I can't tell. In the end I think if the roller chain went from gear box output head block, it would spin too fast. The idler shaft keeps slack smaller and probably takes some strain off clutch.

I assume you test this every month per the standards? Post a video of this from your next monthly test. Find a name plate of whatrigger supplied it and I'm sure I can find out. Does not look like TSC but if really vintage, who knows.
 
Which sort of looks like the SECOA version and the Texas Scenic version and.....I just didn't realize it was a fire safety curtain.

You've been testing it monthly, right?
 
Its not in a venue that I work in, I was taking a tour of a colleagues venue and saw this in the grid and the design of it caught my eye. I am assuming he is testing it monthly. I also think that installation of this unit pre dates when he started at the venue.
 
As far as I can tell from the pictures the jack shaft exists because the sprocket on the clutch has to be larger than the body of the clutch and they needed a smaller sprocket to get the required reduction.
 
I'll bite (but nicely). In a motorized fire curtain setup, which part needs to be tested monthly? The motorized raising and lowering? The manual release handle on the wall? The integration with any automated fire alarm system? The potential fusable links for heat triggering? All of the above?

According to Clancy:
If the curtain system permits operation by power or hand without tripping the automatic descent, perform a full lower and raise cycle. Look and listen for any unusual performance or problems. Deploy the curtain using each automatic release device that is installed in the system, except for the fusible links. You can also deploy the curtain by lifting round weight arbor at the end of the release line, or by disconnecting the release line itself.
A second person should time the curtain as it closes. Most codes require that the curtain close within 30 seconds and that the last 5 to 10 feet of travel must take at least 5 seconds. The curtain must not bounce back into the air after it hits the floor. Travel must be smooth and free of pinch points in the guide system.

If practical and appropriate, test the curtain at least once with the smoke hatches open or with the heat or air conditioning blowers on to determine if air loads prevent the lowering of the curtain. Reset all curtain functions when finished.
 
Which sort of looks like the SECOA version and the Texas Scenic version and.....I just didn't realize it was a fire safety curtain.

You've been testing it monthly, right?
Bill,

This is similar to the fire safety curtain drive at Dominican U in River Forest, IL. It was installed by Vasconcellos in the late '40's. I always assumed that the clutch was being remoted from the head block chain sheaves.

Ted
 

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