Large resaw tensioner concerns

[Aaron Murray]

After appealing to the manufacturer of this saw I have hit a brick wall. So I ask you lovely folks here:

I have a Wadkin resaw and am having some trouble getting proper tension established on the blade. I to recently replace a badly worn ‘saw tension screw’ as well as the Belleville washers used as the tension spring. When disassembling the unit, I took photos of the damaged part and the orientation and quantity of the Belleville washers installed around that shaft so that I would be putting things back correctly. The 1/2" diameter steel shaft itself was tapered within the core of the washer stack and I was told this was not per design and due to 'wear'. All of the documentation I can find pertinent to this saw shows a stack of 18 washers in series. Any copies of the manual for the saw are such poor quality that the stack cannot be discerned including the prinetd one we got with the saw when purchased. The similar C8 saw manual has much better photos and the “top slide assembly” page looks to be identical to our parts. The stack I am referring to is listed as “K12” in that manual from DaltonsWadkin.com.
I have an engineering question about that stack. According to the Belleville manufacturer, the installation is a series stack and follows these rules:

“Single disc springs are assembled ‘opposed to each other’ to form a spring column. This ‘in series’ formation (no.3 in above illustration) is a means of multiplying the deflection of a single disc spring, the force element remains as that for a single spring.”

https://www.bellevillesprings.com/st...-disc-springs/

The part number for the washers in the manual are noted as part number “04020415” and after searching Belleville’s website for a matching part via measurements I found the following washer and am taking engineering data from it:

https://www.bellevillesprings.com/products/d4020415/

At 90% deflection the washer in question will provide 2974 N of spring force (668.58 pounds). That means that the full stack is only returning a little more than 700 pounds of force when fully compressed. Our blades are 2” Lenox with a cross section of .06745 sq in and to generate the minimum tension (25,000 psi) on the blade the stack would need to be returning over 3,300 pounds of force to the upper wheel (1686 pounds to the blade times 2).

The stack of washers is obviously meant not to bottom out since the tension pointer on the saw is using the distance the stack compresses to generate a reading. Is there a configuration of the washer stack that I should be utilizing instead of the one in the manual? Have I missed an important piece of this puzzle?

Thanks.

[Lee Schierer]

It's been a long time since I have even heard of Belleville washers. If my memory serves me correctly you can stack them in series parallel to increase the load. What ever you do don't compress them over 90% as anything greater than that will ruin them.

[Bill Dufour]

I have no idea what kind of saw this is. Band, circular, draw, drag, jig or something else.
Bill D

[Bill Dufour]

Is the spring force direct action or is there any kind of lever action to multiply the force? I see no reason the washers can not be replaced by a coil spring of the correct size and force. Or even an air cylinder.
The washers allow you to fine tune spring force easily. But if you know the range needed a coil spring should be workable.
You say you documented the washers before pulling it apart. But you never said how they were installed on your machine. Just what pictures of similar machines looked like in fuzzy pictures.
How were they actually placed series, parallel or a combination. Was the spring tension correct the way it was.
Bill D

[Rod Sheridan]

Hi Aaron, your arithmetic looks correct, however I’m not convinced the saw was ever designed for 25,000 PSI.

My suggestion is to re-assemble it and verify sawing performance, I think you’ll find it’s fine with the suggested Belleville washers.

Regards, Rod

[Aaron Murray]

It is a resaw. A larger format bandsaw that needs blade tensioning.

[Aaron Murray]

The spring force is directly pressing against the upper wheel of the saw. There is a small lever that tracks the position of the tensioner shaft to display a ballpark blade tension. I don't know if the photo came through OK but what it shows is the stack and condition of the shaft when we disassembled it. The washers were in series. One of the questions I was hoping would be answered here is what should the specs of a replacement spring be (or comparable stack of washers)? I can source a spring with 4000lbs of force but is that how modern band saws (without pneumatic or hydraulic tensioners) are made? If the current stack I have in place flattens at 660lbs, the rest of the force stretching the blade will be via the threads on the tensioning shaft and the frame of the machine with no cushioning. I just can not believe that this is how the saw was designed to work.

[Aaron Murray]

The tension scale on the saw reads from 0-35K and has a reading even higher than that labeled "PBR" and I'm certain it's not referring to beer! That blades themselves are supposed to be tensioned to 28-30ksi per the manufacturer.

[Bill Dufour]

I am not sure if the force has to be multiplied by two or not.
Bill D

[Bert McMahan]

Are the old bellevilles worn out? Can you just use the old ones that you know are good?

Also, different spring materials give different forces, so make sure you're not just matching the size but the material as well. And Belleville Springs isn't the only company to make disc springs (McMaster has a bunch but they're not exhaustive either). That said it'd be hard to get one that's 3x the strength of the standard ones you linked to.

I'd guess the series/parallel idea is the right one. Maybe three parallel, then three series, alternating the whole way through.

You can get a decent estimate of the spring stiffness by looking at the load indicator's travel vs. stiffness. I'm not 100% sure I'm reading the datasheet right but I think the linked washer's travel is 2.65 mm (total height) - 1.5 mm (thickness) * 0.9 = 1.035 mm per washer, or 1.035 mm * 18 = 18.63 mm.

Does the tension scale go from zero to 25 k (or whatever) over 18.6 mm? Sometimes those don't have a "zero" point so maybe just take the travel between 20 ksi and 30 ksi, then back out the travel that way.

Also- does the tension scale include a saw width and thickness? If it's just using the travel of the spring, then it's a force estimate not a stress estimate. Perhaps it's actually reading in Newtons, not in KSI?

Just spitballing.

[Aaron Murray]

Here is the math I am using:
The blade is to be tensioned to 28Ksi (28000 pounds per square inch)
The cross section of the blade = .0355" (thickness) x 1.9" (width from behind the gullet to the back edge) = .06745 square inches
28000 * .06745 = 1888.6 pounds
This is the force in pounds needed to achieve that tension in one cross section of blade.
There are two cross sections being of blade being pulled by the band saw wheel thus the multiplier.
So I need a 'spring' capable of 3777.2 pounds of force.

[Aaron Murray]

The old ones have been tossed. It has been said that once Bellevilles are flattened they lose some effectiveness over time. Since we couldn't get the saw tensioned it was assumed the Bellevilles were shot. Between that and how horribly the shaft within them was worn, it was a good time to get fresh ones. I used the replacements direct from a company that supplies replacement parts specifically for this brand of saw. I am beginning to think there was a major error in the manufacturing process. Partly due to the stack not supplying the needed force but also because the ID of the supplied replacements is 20.5mm (.8") for a .5" diameter shaft. That is a lot of room for the stack to shift and might explain the wear seen on the shaft.
I am glad to hear you mention checking the designed the travel of the tension gauge to the compression of the stack as a double check. The tension scale is in pounds and has .375" of travel to read from 0 to "no blade made can handle this tension". (over 35k) The stack as installed has .8" of total compression before flattening.
We have a tension meter that clamps onto the blade that we have been using to verify our tension is being reached during all of this. I can get it to tension but it is really hard and difficult to fine tune.

[John TenEyck]

Yes, it does, just as a pulley with a rope on each side cuts the pulling force by 2. It's just the opposite.

John

[John TenEyck]

If the spring data is correct, you need 5 washers in parallel and however many series stacks will fit. Five washers in parallel will give you 3340 lbs. Might need to go to 6 washers in parallel for insurance, but it will depend upon how much total stack height you have available. If these washers won't give you the load you need, you might be able to use a stiffer one of larger diameter to ensure you still have enough deflection.

John

[Bert McMahan]

I am glad to hear you mention checking the designed the travel of the tension gauge to the compression of the stack as a double check. The tension scale is in pounds and has .375" of travel to read from 0 to "no blade made can handle this tension". (over 35k) The stack as installed has .8" of total compression before flattening.

What is the reading on the scale at 0.375" of travel? Or, say, between 0 and 0.25" of travel? You will have to match that stiffness for the scale to be right (I am assuming this is the displacement type that measures your spring compression distance, not a direct force measurement type).