Contact Wheel speed

S

Steinmann

Joined
Jan 12, 2010
Messages
99
Dear all.
Are 1270 rpm’s too fast for a contact wheel that is 40 cm (ca. 16”) in diameter, or will it be ok?
Thanks – Michael.
 
If the contact wheel is the drive wheel, that is 5781 SFM....which is pretty darn fast.

The contact wheel is only moving with the belt on many/most grinders ( Those with a drive wheel - KMG, GIB, Bader, etc.), and does not change its SFM ,regardless of size. On those grinders the drive wheel determines the belt speed, and the contact wheel is just turned by the belt. With a 6" drive wheel and a 1270 RPM motor speed, the grinder belt runs at 2168 SFM, regardless of contact wheel size. That is a bit slow for some folks, but will work fine. Usually, there is a three pulley system between the motor and the drive wheel to make the grinder run slow, medium, and fast......or go variable speed with a 3PH motor and a VFD or a DC motor and DC drive.

BTW, most folks get it backward - on a standard grinder, the larger the contact wheel ,the slower it is turned by the belt. It is the small wheels that may run too fast for the bearings, not the big ones. On a grinder running with a 6" drive wheel at 1270 RPM, a 16" contact wheel only turns at 476 RPM, a 1" contact wheel runs 7620 RPM, and a 1/2" contact wheel screams at 15,240 RPM......all while the belt chugs along at 2168 SFM.
 
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Thanks Stacy; The engine is a 3 hp 2850 rpm beast... The contact wheel will be self made – hence the question. I can go down with step pulleys but I want to stick with my (limited) budget and if possible – avoid them. I made the drive wheel as small as possible and then came out with these calculations (1720 rpm for the contatwheel, which is not the drive wheel).
Said this, what do you think should the recommended rpm be for a 16” contact wheel? Stock removal use only.
Thanks again and a lovely week everybody – Michael.
 
It is the speed of the belt that is important. Something in the 2-3000 range would be my preference for single speed.

What size will your drive wheel be, and do you plan to drive it directly or by belt?
 
Hmmm… good questions. That’s the kind of info I am looking for :). As mentioned, the wheel is already crafted – 16” (40 cm) in diameter (plywood). The macro idea is to use the engine’s weight as tension for the abrasive belt, to use a drive wheel (size?) on the engine that will drive this contact
wheel directly. This way I will have the low budget I’m looking for. I’m also trying to avoid the use of step pulleys.
BTW, do you mean that 3000 rpm's will be ok for my 16" contact wheel to spin?
 
I would make sure of two things before you crank that thing up to 3000 RPM. First, make sure the wheel is balanced and second, I'd want it mounted on decent bearings.
 
Ha, I originally wrote "stop thinking about rpm" in my previous reply then I removed it as it seemed mean

STOP THINKING ABOUT RPM OF YOUR CONTACT WHEEL! (except see caveat below)

You need to decide how fast you want the belt to move. I suggest 2-3000 surface feet per minute, that is the belt travels 2000 feet in one minute. Once you decide this speed you then calculate how big your drive wheel should be.

An example,

My motor is 1725RPM. My drive wheels is 3.375 inches in diameter. I calculate the circumference, pi * diameter = 10.6 inches

10.6/12 = 0.883feet

0.883feet x 1725RPM = 1523SFPM that is, circumference x motor speed = belt speed.

You will do the reverse calculation, you will decide the speed you want the belt to move and then work back to the drive wheel diameter necessary to achieve it.

I use this speed on my grinder for all operations as I have no speed control. I think I would prefer it be a little faster if I could choose my speed.

I would love to have 1000 sfpm, 2000 and 4000 if I had step pulleys. High speed for rough work and slower for finishing.

http://vintagemachinery.org/math/sfpm.aspx

As Stacy explained your contact wheel will turn at a much slower speed. I suggest you read his explanation again. The large contact wheel will turn much slower than the smaller drive wheel (determined by the ratio of their diameters). Since you have a homemade wooden wheel this is a very good thing.

I think maybe the full weight of the large motor will be too much for the abrasive belt?
 
Erenestrome is correct, only focus on SFPM.
TO put things in perspective, I run at 5500 SFPM on my flat platten and 10" contact wheel when using ceramic belts to hog.
I run 1/2 that or slower when using finer belts on my variable speed grinder.
CW
 
OK, you have given me the info that would have helped my calculations in the first post.
A 16" wheel is approx. 4.5 feet circumference. If you want the grinder to run at 3500 SFM, that will make the wheel turn at 775 RPM. If the motor turns at 2850 that means the drive wheel should be 4.35" diameter.

The 4.35 drive wheel is not a problem. Turning a home made wheel at 775 RPM is probably OK if it is well constructed and balanced. Running it at 1270 RPM may be unwise.

My personal comment on home made contact wheels is :
Why would you scrimp on the one component of the grinder that matters. If the wheel is the least bit out of round, off center, out of parallel, etc. - it may shake the whole grinder to pieces...at the least it may make grinding nearly impossible.
Spend the $500 on a good contact wheel and bearings, and make the grinder out of an old bicycle frame.

Second is - Do you need a 16" wheel. I have one that I almost never use. Unless you are doing hollow grinds on large blades, it does nothing the a 10" wheel won't do.
 
Also, you may find that if you are relying on the weight of the motor to tension the belt, your options for tracking may leave something to be desired. Normally a tension wheel is combined with tracking control. If you have only two wheels, the drive and the contact, one of them will have to be able to pivot with adjustment screw(s). Normally with an upright two wheel grinder, the contact wheel is the drive wheel, leaving the top idler as tension/tracking. Mounting a large motor in a swivel plate for tracking probably won't work too well, and using a 16" contact wheel to control tracking may not work well either.

Working on a grinder with fiddly tracking really sucks.
 
I think one thing to consider is this:

I have no blade grinding experience, or even grinder building experience (Yet). I'm currently building a grinder thats 2X72. I havent finalized ratios from motor to belt or anything yet. But I have started on the wheels. 3 wheels for my grinder with a vertical grinding area and the tensioner set behind the main wheels. I'll post pictures and explain how it works when its finished but the wheels will not change, the platen will change depending on the grinding to be achieved. Right now, my idler/tensioner/tracking wheel is 2.875 inches diameter 7075 aluminum 3.25 inches wide. It is mounted on ABEC 3 bearings rated to 10,000 rpm. A ground and hardened 5/8 inch shafter runs through it. I spent 6 hours total machining and assembling that one pulley. It is 100% round, true, balanced and the bearings are loosely pressed into the pulley (can be pulled out, but wont/shouldnt move under normal conditions. plus the bearings are surrounded by collars and washers so falling out is impossible.)

I have 10 hours in each contact wheel (one drive, one idler). 7.5 inches diameter (largest I can fit on my lathe)

The wheels are a combination of MDF, Stainless steel, and aluminum again 100% balanced true and round.

I will probably start on the frame sunday and see if I can get the drive wheel mounted. My whole grinder thus far has only cost me the cost of bearings. I'd scrap the plywood wheel and either buy a aluminum one, or I can send you the design for my wheel (PM/email me from my profile if you want them). They are the most important part of a grinder hands down and need to be as close to perfect as possible. If you can get the wheels made, you are 50% there. Hope this helps focus the importance of good wheels. Good luck.
 
I machined my tracking wheel and platen idlers out of UHMW, and press fit bearings into them. They work very well. Cost me nothing. Saved me a lot.
 
while i woudl love a big wheel 24 ich i guess woudl work but 36 would be better for me i would never think to make one here at the house for fear of it coming apart while spinning (i would be using it for sushi knives mostly but it also make a cool grind on a bowie )
 
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