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Thread: Bent forklift fork question
02-24-2005, 11:58 AM #1
I still can't believe it, but I somehow managed to bend a fork on my 20,000 lb forklift. It's not that bad, not noticable just to look at them but just enough to cause slight "rocking" when picking up machines which bases extend out near the tips. Six foot forks, the bent one is 1/2 inch lower at the tip.
What's interesting is it's not bent at the heel but bent about 2 feet from the tip. Matches the other fork perfectly up until that point.
In trying to straighten it, I blocked the mast stage with a heavy timber so it couldn't rise, centered the offending fork on the carriage and used a 25 ton jack to bend the fork tip upward. The result was the fork did indeed bend upward but at some point it raised the entire front end of the (32,000 lb total weight) forklift ! So it didn't bend it "enough" to do any good, as it sprung back to the same bent position. Not to mention the fact that even if it had worked there is always the possiblity it would have bent the whole fork at the heal, rather than where it really needed to be bent two feet from the tip.
So, what I'm wondering is if using a torch to heat the bent area, combined with the 25 ton jack trick, might be best in this case. I wouldn't even think about that approach if the bend was at the heel, way too risky...but 2 feet from the tip seems like it wouldn't be "dangerous" for future use but worse case senario might rebend easier than it would have before, but most likely would be of similar strength to original.
Btw, these forks cost nearly $4,000 per set new, so replacment is out of the question. The other alternative is a guy in Michigan recommended by a "big 'un's" forklift dealer, with a 1,000 ton press who says 'no problem, do it all the time, 100 bucks...but shipping from
SC will kill ya'
02-24-2005, 12:19 PM #2
How much is it worth to you to be able to move machines, some that costs a hell of a lot more than $4K, with confidence? If that value is more than $4K then buy new ones, if not, put them on a truck.
02-24-2005, 12:23 PM #3
I'm no engineer and couldn't tell you the exact force needed but I would think it would be a heckuva lot less than 1,000 tons if the bend point was set up between blocks. I've seen forks off of an 8,000 lb. Hyster straightend in a 50 ton press, cold with no problem at all. Granted your 20K forks are a lot beefier but that's a big difference in force.Is there a shop with a decent size press closer than the one in Michigan?
02-24-2005, 12:35 PM #4How much is it worth to you to be able to move machines, some that costs a hell of a lot more than $4K, with confidence? If that value is more than $4K then buy new ones, if not, put them on a truck.
Re not needing 1000 ton press, of course not, I just mentioned the tonnage to give some idea of the scope this guy is used to working with [img]smile.gif[/img]
02-24-2005, 12:40 PM #5
Don, I'd be afraid to put any heat on it whatsoever. Assuming they're some sort of heat treated alloy, you'll have to heat to a far higher temperature to get any appreciable help in bending than they would have been originally tempered at, so the resultant material would approach fully annealed at that area. They're designed to work as a cantilevered beam of somewhat constant stress. The taper along the length keeps the working stress higher as you move out along the length than one might think, so you could end up with the fork bending again at that point with far less load than it took to bend it this time. Cold straightening in a hydraulic press is definitely the way to go.
So much for the why. IMO, there's no way this would take a 1000 ton press. Based on some of the things I've straightened in our 85 ton press, I'd say anyone with a 150 ton can straighten it with no problem. Between Charleston and Savannah there oughta be at least a couple dozen shops with a press this big or bigger. I'd check first with anyone who does hydraulic cylinder repair work, since most anyone who's set up for doing good sized cylinder work would normally have a press big enough to straighten 5 or 6 inch cylinder rods.
Just remembered something I tried once a while back and took some measurements. Had something we were trying to press apart on one of the cranes that couldn't be carried to a press. Used a 100 ton ram and a bottom plate attached with 4 large pieces of B7 rod to capture the part we were pressing on. The plate was attached on 12" centers, and was 1.75" thick and 11" wide. Load was at the center of the plate, and the 100 ton ram put a permanent 1" bow in the plate without ever moving the part. Based on that, and guessing as to the size of your forks and their yield strength, I'd say a 150 tonner will do it easily.
02-24-2005, 12:47 PM #6
Cliff, I was against the heat bend idea until I called a shop in Savannah that says they have been sucessful doing it that way. But they were mostly thinking about the forks from smaller trucks, and "sucess" in rebending may be very different from the "sucess" their customer's had after they got it back !
Re 1000 tons, see my second post.
But speaking of tonnage requirements, I once bought another smaller 5K forklift that came with badly bent (5 foot) forks...3 inches difference at the tips. In this case the bend was at the heel. Anyhoo, I found one problem in attempting to bend back simply how to actually fixture the darn thing in the (typical shop H frame 35 ton) hydraulic press. I eventually hit upon just driving the forktruck to the press and being a Jet make with the open sides, potruding the fork thru the side and fixturing blocks under. In spite of this, I was not able to bend it enough to effect results.
The solution was to turn the fork upside down on the truck carriage and then basically reverse the process that bent it in the first place. End result has been fine, have loaded the forks full load many times and the rebent fork still matches the other one perfectly.
And before anyone asks, I can't turn the fork upside down on the Hoist..different mount situation.
02-24-2005, 12:59 PM #7
Those forks are heat treated. If you put in enough heat to make bending easier, you wreck the temper and will end up with a weak fork that will bend easier in the future. I'd do the cold bending only and then consider a magnaflux check after the fork is straightened to be sure no cracks got started.
02-24-2005, 01:04 PM #8
Not knowing the geometry of these forks, lets say the cross section was 1.5x6". Simply supporting the load (bend in the center) so a 4 foot span. Also assuming this is 1020ish steel with a yield of 36000psi, you will need 81tons to straighten it.
This is ignoring the fact that they might not be 1020 steel....and localized strain hardening. Becareful if you attempt this yourself to concentrate the force at the bend, so you dont end up yielding areas around it. Ever try to bend a paperclip back EXACTLY where u bent it the first time?
With 81 tons....I think Id let the pros do it! And somehow I remember being told that forks were heat treated, or at least a steel with higher hardenability, if thats so, be VERY careful with a torch, and let the suckers cool SLOWLY! dont happen to have a hardness reading on those forks do ya?
02-24-2005, 01:20 PM #9
For an 'on site' repair suggestion - Do what you described in your first post, but put 20,000 pounds or more on the forks first, between the bend and the heel, before you block the mast and start jacking. Putting cribbing under the counterweight so more of the machine weight comes into play rather than pivoting around the steering axle will also help, right?
02-24-2005, 01:21 PM #10
PS - I straightened the bent fork on my 5K lift with a 50 to press and I don't remember what tonnage it took, but it wasn't anywhere near 50 tons.
02-24-2005, 01:24 PM #11Do what you described in your first post, but put 20,000 pounds or more on the forks first,
02-24-2005, 01:27 PM #12
Can you chain the fork down to your loading dock and tilt the mast forward?
02-24-2005, 01:32 PM #13
Oooh...I like that thought Mud as the tilt cylinders are hellaciouly powerful and indeed double acting. But, can't think of anything to chain the tip to...dock is just all concrete affair. I need another Hoist 20K that I could chain the fork of this one to the fork of the other one !
02-24-2005, 01:39 PM #14
A great big scrap I beam, or an old flat deck semi trailer?
02-24-2005, 03:11 PM #15
I've got the press, 150 Ton Dake,
however no experence.
Be careful about chaining.
Once pulled a 3/8 chain apart with a port-a-power and the broken link smacked the wall 50 ft. away.
Leason experenced and remains fresh in memory.
Thankfull no coworker were hit.
02-24-2005, 04:01 PM #16
BEND THE OTHER FORK TO MATCH THE BENT ONE!!!!
May try to peen the under side of the fork with a large ball peen hammer (jack hammer?), this works on straightening gibs and other long length parts.
I know a couple of shops in the Houston area that have 500 ton presses that can straighten the kinks out of 11" OD solid 4145 HT bars.
02-24-2005, 04:14 PM #17
Would you straighten a bent barrel on a 30-06?Why not, it would save hundreds of dollars! It probably will never blow up in your face... (help, I'm blinded! AAAAH!)
Between tempering, metallurgy, cold-working, etc-there are WAY too many unknowns.
Just because it CAN be done, does not mean it SHOULD be done, or that it's even ADEQUATE!
I could see doing this to a snowplow, maybe...
Does the manufacturer approve of this solution? Probably not: so why should YOU assume such a liability?
How much is safety actually worth?
Drop one $4,000 machine with your $100 repair and you will have paid $4,100!
When somebody gets injured or killed because of that blade, you will wish you had paid the money...how much is a class action suit worth? Workman's comp?
What do you tell the guy's widow when she asked "how did this happen?" What do you tell your wife and kids?
Most good lawyers start with a retainer around $5,000. PLUS if there is an accident, you have to buy the new blades anyway. Now you are out at least $9,100....
Just buy another pair, and keep the other blade as a "spare"-worth $2000 when you only need one, right?
Sorry to vent so much spleen, but things like this really worry me because if effects peoples lives and limbs...nothing personal.
02-24-2005, 04:35 PM #18
Neil, if you actually read my post you'd know that the bend is 2 feet from the fork end, so 4 feet of the fork is as new. Straightening it back is more a convenience thing rather than a safety thing...will keep me from having to put a wood wedge in when a machine rocks a little. Except for the one exceptional instance that bent it, it's quite rare for there to be much pressure on the fork ends. And as previously mentioned, I've done it myself on the 5K lift with excellent results.
From talking with numerous "large lift" folks involved in the trade for decades, who have rebent forks I'd put the odds that controlled cold bending it back will contribute to failure as miniscule.
OK, come clean..you and jkilroy used to sell life insurance or minivans didn't you ? "well, m'am, just how much is worth to your piece of mind to..."
02-24-2005, 05:05 PM #19
don not dave;
are you close to a railroad siding?
maybe one of those GE locomotives would weigh enough.heh heh jim
02-24-2005, 05:29 PM #20
how about constructing something like the blue structure in the picture below out of some steel. You could use an I-beam for you the top and some thick plate for the sides. It could probably be built beefy enough to take the force, the question is, is it worth the time and cost of steel.