Direct drive KMG with LoveJoy coupling

JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Knifemaker / Craftsman / Service Provider
Joined
Jun 11, 2006
Messages
8,633
I have been fighting the direct drive bandwagon for a long time but my link belt broke a little while ago and I have tryied a few off the shelf belts and am not happy with the vibration. The problem is where the belt is spliced it's just a smig wider and stiffer and when it goes around the small pully it pulls on the motor. With the link belt it was fine. But direct drive was out of the question for me because I could not get the belt speed I wanted/needed. Well after looking through my VFD manual it will take my motor to a whopping 4x speed if I set the vfd to 240hz max speed. That combined with my 5" drive wheel will give me just over my desired 9,000sfpm. Now we are talking but I don't really want to replace the back of my KMG and drive right off the motor as it will be going quite a bit over the rated speed. So my though was to use a lovejoy coupling to couple the motor to drive shaft. This alows direct drive and still isolate the motor from the belt tension and at the much higher rpm I'm thinking that will be good. This is the coupling I'm thinking of using. Any thoughts on this and also over driving the motor by that much, it's not like i'm going to be running that fast all the time but I like to have it when I need it.

jaw_type_coupling.png
 
I'm guessing you have a 1750 rpm motor? and you're wanting to turn it to 7,000 rpm? That's a tad fast for the motor - not sure how it will hold together. I think most standard motors are balanced for a max of 4,000 to 5,000 rpm, more like 4K. Loss of torque will be a good bit. Have you ever turned a belt 9,000 SFPM? That's HUMMING!!! I'd sure hate for one to break on me at that speed.

Also, just how much experience do you have with Lovejoy couplings? Those are on the tricky side to make sure they are lined up VERY close.

You might consider removing the shaft assembly/bearings, etc, bolting the motor direct to base plate, put the drive wheel on the motor shaft.

Ken H>
 
I have used this type of love joy and really like them. Thy don't have to be aligned perfect and work very good.

s_flex_web.png
 
If you have a 1750 RPM motor, you can almost certainly run to 120 Hz/3500 within the design parameters for the motor (it will have been designed for 2-pole windings, as that's the fastest, then fitted with the 4-pole windings to give 1750 RPM). Significantly faster and you are on your own.

At 240 Hz, I'd expect the losses to be huge and the torque to be almost non-existent. Even without the extra losses, and assuming constant power, you'd be at 4 times base speed and 1/4 of the base (rated) torque. With the additional losses due to running so far from the design frequency, I'd expect significantly less than even this in the real world.

High speed spindle motors are available that are specifically designed for high frequencies (400 Hz/24000 RPM is pretty common on ebay), but they are a lot different to a standard industrial motor.

Personally, I'd run to 3600 RPM or thereabouts and change the drive wheel to suit. I'd probably do as others seem to have done and go direct drive. A new drive wheel to fit the motor probably won't cost much different to a drive coupling set.
 
Maybe I'm missing something, but why not just buy some new link belt? Or, unless it is worn out completely, you could just link the old belt back together a little shorter. If it was working for you, it would seem to be much less trouble than re-mounting the motor, over revving the motor, etc. Apart from the issues already mentioned, the motor bearings may not be able to handle 7,000 rpm for very long. If there's no industrial supplier nearby, McMaster Carr carries all types of link belts, and I'm sure a lot of other online suppliers do too.
 
Yeah the link belt is toast, tryied going shorter and no luck just broke again. I can get another one but was just wondering just wondering if this was worth trying to go direct drive.
 
I suggest you call and check the rated speed of the coupler. IIRC, they are not rated for high speed. Additionally, set the VFD for 120Hz. No one needs 240Hz feed to the motor on a grinder. Also, the grinder itself won't do well at that SFM.

I know you are a bit "what the heck" at times, but running a grinder at that speed is crazy ... even for you :)
 
Yea, I'd wondered about 9,000 sfpm myself - isn't around 7,000 sfpm the max the belts are rated at?
 
Your motor's bearings are likely not rated for 6k RPM. Also, the motor's cooling fan will sound like a VSTOL jet at this speed.

The belt vibration is odd -- many power transmission systems are engineered with belts in order to isolate vibration between the drive side and the machine. Have you tried a new Gates AX series belt? If your sheaves are small diameter, then try a cogged version instead of standard. You really shouldn't have a problem with vibration due to a belt system unless something is wrong (worn belt / worn sheaves / bad bearings / bad shaft machining / poorly aligned sheaves / etc.)

But otherwise, yea...lovejoy couplers are great. If you look in the mcmaster catalog, you'll find that they're made in many configurations, some of which have a damping effect on vibration.
 
I think on of the bigest factors is how small the sheave is on the KMG. This is a KMG clone that I built so everything was machined as square and true as I could make it. But with it being so small I'm thinking the belt has a hard time running around it and flexing to fit. And when it gets to the slightly stiffer joint it yanks the motor. My easiest option would be just another link belt. I might look at increasing the sheave on the drive shaft as it is rather small. When I made that sheave I made it as small as I could but I think I'm to small. Right now to hit 9000sfpm I'm only over driving it to 91.7HZ. But if I bump the pully up in size and increase the HZ I will still have my max that I want and more tork in the lower speeds. I know 9000sfpm seams crazy but honestly I have not had a single problem with any belt that I have used at that speed. I know there are commercial grinders like the square wheel that do 6000-7000 and so if I was going to drop my max speed I think that would be my limit to drop to.
 
I'm not a fan of link belts, esp. when vibration is an issue. Gates makes several V-belts specifically designed for small sheaves and reducing vibration.

Here's one:
http://www.gates.com/products/indus...eavy-duty-v-belts/super-hc-molded-notch-belts

Basically, use an AX series belt that is cogged with machined sides. The cogs let the belt bend around a small sheave much better than a normal V-belt. Give their engineers a call -- they have other models with vibration reducing fabrics woven in to the belt.
 
I'm with Catalytic on this one. The only reason to use link- or jointed-belts is when the design of the machine makes it impractical to use a continuous belt.

The Cogged or moulded-notched, Raw-Edged belts will give better performance under any condition I've encountered, as long as you can actually fit them. Most of the claimed superiority of link belts is when they are compared to the vee-belts that were current technology back when they were developed. The Wedge belts that are current technology today are so far ahead of the old Vee-belt technology (which dates from the 1950s) that comparison is pretty meaningless.
 
Trust me, if you've ground at 5000 sfpm you will realize you dont need any faster than that.

My KMG with direct drive and frequency doubler to get 3600 rpm from the motor shaft on my 3hp 1800 rpm motor will do 5000 sfpm and it's blazing fast at that speeds. There's no need to go 2x as fast as my high speed in this video.

[video=youtube;EdtC2FIr_aA]https://www.youtube.com/watch?v=EdtC2FIr_aA[/video]
https://www.youtube.com/watch?v=EdtC2FIr_aA
 
I first used a 4 inch drive wheel on my grinder for about 3690 SFM and I just had to have more speed so I had a drive wheel made and crowned at 4 3/8 inches for about 4000 SPM but I thought that I needed more speed so I found a 5 inch drive wheel for about 4500 SPM and now I find myself turning down my drive to about 4/5 power for about 4000 SPM tops and much less for the finer grits while finishing. I am no longer in a hurry and I think that I have more control over grind lines and smooth beveling with a lower speed. Of course the higher speed is helpful when profiling blades with a 36 grit belt but the grinding would get done in a few more minutes anyway. The slightly lower speed has got to be easier on the equipment and is probably a little bit safer too. Having more torque at lower speeds while using a flat platen is a slight benefit also. I think that 5000 SFM is about as fast as anyone really needs... Just my opinion again. Larry

CCkB6oLm.jpg
 
I get what everyone is saying and I agree faster is not necessarily better or faster. But for me when I rough in the profile and edge bevels I really like the faster speeds. It is very smooth at fast speeds, but things can also mess up fast. I would say most my grinding is between 4000-6000 unless I'm using gator belts and thoes are 1000-3000. But I tell you what you pop on a blaze belt and crank it to 11 and oh man it flat out removes material. But hell the norax finish belts are rated to be ran at 6,300 SFPM and Norton aluminum oxide rated at 7500 sfpm. Could not find a rating for the blaze belts but have seen numbers tossed around that are higher then there aluminum oxide. So as I said befor it's just a occasional use at the high speed but nice to have. Here is a video I took after some tweaking and reprograming. Seams to run good but will defently look into thoes better belts

[video=youtube_share;n4C0e79MMPM]http://youtu.be/n4C0e79MMPM[/video]
 
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Nice video - I really like the balancing quarter trick. I've got a video somewhere showing my grinder running at 4,000 SFPM with a quarter balanced on edge.

4,000 sfpm will do some grinding, hard to imagine grinding at 9,000 SFPM! I'd like to watch that..... from a distance :)
 
That video is shot with the grinder going 11,400 SFPM. I will toss a blaze belt on it and shoot another video of it grinding. I actualy called Norton today and talked to one of the tec guys for about 1/2 hr about my set up. I expressed my concernes about the belt integrity and grit stripping at these high speeds. He said the seam is very tough and if it comes apart to call them because that's a quality concern. About grit stripping he said thy addressed that when thy went to the plus version of the belt which has thicker coating. He basically said in a nut shell that thy will take anything I will throw at it. I was concerned with belt life decreasing with the higher SFPM. He said that will have to be determined by my uses as each use is different. But to keep track of amount of blades ground and then adjust speed and compare. But other then that he said thy are crazy tough belts and designed to be used very hard to brake down the grit. One sign of order driving the belt is it not cutting as well and actualy burnishing the surface, the belt is going to fast to allow the grit to cut and it just rubs. This also causes heat which can build up metal onto the top of the grains which stopps the cutting. So he said basically if it's cutting good then I'm good.
 
I just watched the video on computer, before it was without sound or anything. That belt is humming! I also watched the other video where you first built grinder and did some grinding at 9,000 SFPM - that does throw some sparks! IMPRESSIVE!!!! When I saw the jeweling on the parts I remembered when you first built that grinder - I was impressed then, and am still impressed.

Keep up the good work.

Ken H>
 
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