Tuning fork to stop engine vibration?

[oldunion]

I have a 62cc engine on the back of a scooter frame made for an unmodified 41.5 cc engine. The engine is putting out a ton of power for what it is, 8hp maybe from 5 stock, and i sheared two of the 4 engine mount bolts right off. I am going to tap out the threads to accept a bigger diameter bolt, use an engine mount with springs and rubber grommets to reduce engine vibration, and id like to work a tuning fork into the equation.

I remember some hammer company put a tuning fork into their handles to stop the impact tremmors when the hammer would strike a surface. I am thinking if i put a tuning fork on or near the engine perhaps attached to the frame, this will help my cause. True assumption? if so what frequency tuning fork do i need.

My engine revs right now to 12,500 rpm, but may soon go to 20k. As i said, about 7-9 hp. engine mounted to the frame via 4 bolts only.

 

[Cliff_J]

A hammer is going to have a single resonance with a high Q that is relatively easy to damp with a tuning fork. I watched a History Channel show where they detailed the construction of that hammer, and what they showed on the oscope screen was a surprisingly simple sine wave decay. It might have been only for illustrative purposes but regardless its a far simpler example.

The manufacturers of cars/motorcycles use isolation (rubber mounts) to minimize the transfer of vibrations and counter rotating shafts so that the counter vibration is in step with the RPM of the engine. Even if it is predominately one frequency you feel being excited that doesn't mean that the engine isn't producing an RPM depedant frequency doing the exciting.

Grommets aren't going to do much if you have a "rigid" engine mount. Like all these custom Harley chopper TV shows where the rigid engine mount hard-tail design is popular, better get some thread locker to assemble all the other pieces...

 

[FredGarvin]

First of all, why are you looking at vibration? If your engine is putting out that much power, shouldn't you be first looking at torque issues? I'd say that increasing the beefy-ness of the engine supports is the first task. Are you feeling an unacceptable amount of vibration during the running of the engine? Is it over the entire range of operating? There are a LOT of things to look at when tuning a system for vibration response.

With an engine and frame you are going to have so much broadband noise, it is very tough to tell you what frequency to go after. You could have a resonance in a structural support, etc...I would say to look at the basics first. Cylinder frequency, the 1 per rev on the crank, did you check the balance of the rotating components in your engine and wheels...the list goes on and on.

The company was Stanley that put the tuning fork in their product. That was a solution to a specific problem though. I doubt you'd be so lucky as to do the same. If you were, all cars would have tuning forks of some kind on them.

 

[Cliff_J]

On the rear brakes on my Firebird the brake caliper has a bolt on ~2lb iron dognut listed in the repair book as a damper. That's for a rear disc brake, imagine the static mass needed to damp larger vibrations.

As FredGarvin pointed out, torque is a likely culprit for the broken bolts. Or possibly a misalignment on the mount causing extra stress, or the new engine weighs significantly more than the old one and stresses those bolts more, and so on.

 

[oldunion]

The engines are about the same size, but as i stated, we are going from less than 5 hp to 8 or more. So, i guess torque is the issue. However, my point in killing vibrations is that even with threadlock and a basic vibration killing system via springs, the bolts manage to unthread themselves enough to allow for torque to finally finish them off.

essentially what the springs do now is take the vibrations from the four engine mount bolts and sends them directly to a different spot on the frame which is away from where the engine mounts up. This is with rubber grommets. Two of the bolts that had the system, were still there but badly stripped. The other two without the system (because it could not fit) were sheared as i said.

I am definitely going to tap the current holes and rethread them for the next size up, in coarse instead of fine thread. But i feel there is more to fixing this issue than the bolts. So a tuning fork wouldn't help at all?

 

[Cliff_J]

Think about hauling around a bowling ball in the back of a pickup truck. If you had it secured down you could drive like a wild man. But what if it was loose and you floored the gas and then slammed on the brakes? Way overdone but the idea is that the impact if going to subject the truck to higher forces because its a sudden change instead of a gradual one.

If the springs/rubber are too soft, the engine will bascially slam back and forth and this increases the forces tremendously. Its what an air powered impact wrench uses to generate huge torque yet you can hold it in your hand. If you can't stiffen it up then a rigid mount would couple the engine to the frame and eliminate the impact of it shifting around.

Also, with only two bolts rigidly mounted the engine is swinging on those bolts, and after they bend back and forth a small amount they will work harden, become brittle and its easy to understand why they would break. All four should be rigid to avoid this, then the motor is just trying to stretch the bolts or push against the frame.

 

[oldunion]

Ok, the stock arrangement is as such: 4 bolts. the two bolts nearest the drive clutch are flush with the frame. the two furthest away near the pullstart are raised maybe half an inch on aluminum spacers, again this is stock. What i did was put rubber grommets under all 4, and put springs around the outer two spacers to diffuse the vibration from the engine right into the rubber grommets.

are you saying that by using the rubber grommets i am actually losing the battle against torque destroying my threads? Should i just use threadlock and bolt it down tightly to the frame, metal to metal...? maybe nylon washers that can be tightened down and remain flush more than grommets?

 

[minger]

I was thinking the same thing. In cars, if you get wheel hop or unnesessary engine vibration, you typically remedy that by stiffening up the motor mounts (polyurethane, solid mounts, etc). By softening up connection from the motor to the frame, you are allowing it to move back and forth, which I would think would make things worse.

I think your best bet is to lose the rubber/springs, and look for a higher grade bolt, and/or consider fabbing something up so you can add more bolts to connect to the frame.

 

[FredGarvin]

I am definitely going to tap the current holes and rethread them for the next size up, in coarse instead of fine thread. But i feel there is more to fixing this issue than the bolts. So a tuning fork wouldn't help at all?

Since the last one's stripped, I would go with the next larger size and a fine thread fastener. The fine thread fasteners in the UNF series have a larger thread area and thus a greater capacity for load. I would also look at the quality of the fastener. If you can find them, I would recommend going with a grade 8 bolt. A better bolt will allow a higher torque and thus a greater preload on the bolted joint for increased strength as well.

 

[oldunion]

Since the last one's stripped, I would go with the next larger size and a fine thread fastener. The fine thread fasteners in the UNF series have a larger thread area and thus a greater capacity for load. I would also look at the quality of the fastener. If you can find them, I would recommend going with a grade 8 bolt. A better bolt will allow a higher torque and thus a greater preload on the bolted joint for increased strength as well.

It makes sense. Let's see, at my disposal i have ace hardware, home depot, and napa auto. Polyurethane bushings would be insane! where could you get 4? And if i can only find a tapping kit in coarse thread, would you recommend holding off till i find fine?

 

[FredGarvin]

I think since you're increasing in size you're going to be OK. Just keep the threads in the back of your mind if it is still an issue down the road.

If you could wait a day or so, have you ever discovered the wonderful, delightful world that is McMaster-Carr? The serious toy store for our types. You could look into the hardened socket head cap screws. I use them all the time on test rigs. They are worth their weight in gold. They are inexpensive and rugged. I am sure you could find some good support mountings there as well.

Try going to: http://www.mcmaster.com
On the left side you'll see a search box. Look up "SHCS" and select the "standard" under the head style. It will look like this:

 

[oldunion]

thats an awesome resource but unfortunately i can't wait. itll be stainless and ill use teflon tape and threadlock.

 

[faust9]

Don't use teflon and thread lock together. That's like trying to superglue and egg to a non-stick pan. Don't bother with soft bushings of any sort. Use Al inserts if things are loose otherwise just bolt everything together. You should have a bolt distributor somewhere in your area if you can't find a dedicated bolt distributor (you can get all kinds of goodies such as steel threaded inserts with flanges which can be used in place of rethreading your frame or grade 5,8,8.8,9 bolts and the like) if not, NAPA auto carries grade 5,8, and some grade 9 bolts.

If you have room(I don't know if your bolts extend all the way through your frame) then you might try some nylock nuts on the opposite side of the frame--kind of like a jam nut. Nylocks are easier to remove than bolts with threadlock(red threadlock gets extremely hard--a sloppy rethread means you'll end up with lots of hardened 'glue' in your threads); however, you shouldn't reuse nylocks once the nylon has been cut. Well that's not entirely true, you should verify the running torque of a nylock if you plan on reusing it but you probably don't have enough threads extending out the back side(if any) to do this.

[edit] Googled bolt and Charlotte NC and this came up.
http://www.google.com/local?num=100&hl=en&lr=&safe=off&q=bolts&near=Charlotte,+NC&sa=X&oi=localr

 

[oldunion]

http://www.davesmotors.com/store/product891.html

thats my engine. I should have specified...there are no nuts. the bolt goes up through a hole in the frame and tightens the engine down on the frame, with thread alone. there's no nut on the other end. its testy too, when a bolt breaks it flew up into the flywheel once and ruined my riding for a long time.

ill check napa auto this weekend for grade 8