Geoff Weeks Posted June 26, 2023 Share Posted June 26, 2023 My question is how long has it been known? I was chasing a vibration problem on my 1942 IHC. 2 piece driveshaft (jack shaft and drive shaft) and found the rear had a lot of run out. Looked way too small for the size truck (2 1/2" tube) and too long for that sized tube. BUT that is what the factory manual calls out for that wheel base. I ran the numbers through Spicer's calculator and sure enough it's operating range included running at critical speed for the shaft! Long story shortened a bit, I am getting a new shaft made that is 3 1/2" shaft, and converting to modern joints. The front jackshaft is short enough not to run into critical speed or even 1/2 true critical. My other almost identical truck with a longer wheel base has a longer shaft that flares from 2 1/2" at the joints to 3 1/2" in the middle. That is also what is called for in the manual and also runs well into critical speed according to calculations. I will have to add a 2nd jackshaft to keep the driveshaft short enough to keep away from critical speed. So, did they not know about this "back in the day"? Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/ Share on other sites More sharing options...
doubleclutchinweasel Posted June 26, 2023 Share Posted June 26, 2023 I find studies about driveshaft critical speed dating back to the 1960's. Not sure when they started thinking in those terms. Every rotating device has a critical speed, though. That is a fairly fundamental engineering principle that gets applied to most anything that rotates. So, it IS kinda weird that those left the factory that way, isn't it? I'd be curious to hear what you, or someone else, figures out about this. 1 Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533622 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 As someone who was a mechanic. I can look at something and say "that isn't strong enough" but can't tell why and when it would fail. Not an engineer. However I was always surprised by the skinny little driveshaft on a truck that have 21500 GVW. When you add in the O/D in the main transmission, I just knew I wouldn't be re tubing the driveshaft back to stock size. Both my K-7's have the stock drivetrain. When I started researching it I found something interesting. There are many "charts" on the internet listing critical speed for drivelines, most aimed at racers. (afterall, most would think the OEM did their homework). When I found Spicer's calculator on the net, it show much more conservative values for max driveline speed. I think the difference is between "all day everyday" usage and racing. There was also something listed that I hadn't heard of "half true critical speed" which was safe to pass through, but to be avoided as cruise speed as a 1/2 harmonic can be troublesome. So I am ending up with a much larger shaft (but aprox the same length). The U joints on my shaft now were obsolete by the end of the K's run (late 40's) and although mine are still ok, it is not possible to find replacements. If a new shaft is going in, new yokes for available joints only makes sense. 2 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533623 Share on other sites More sharing options...
terry Posted June 26, 2023 Share Posted June 26, 2023 What is critical speed? never heard that term. terry 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533624 Share on other sites More sharing options...
doubleclutchinweasel Posted June 26, 2023 Share Posted June 26, 2023 I have worked a good bit with balancing machines in use on some of our products in the past. The balancer manufacturers mentioned that half true critical speed. They said the same thing...you can pass through it, but don't hang out in that neighborhood. I've heard that Francis wheel turbines in hydro plants experience a vibration at some RPM while ramping up and ramping down, and that is probably that same phenomenon. 2 Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533625 Share on other sites More sharing options...
Popular Post doubleclutchinweasel Posted June 26, 2023 Popular Post Share Posted June 26, 2023 2 minutes ago, terry said: What is critical speed? never heard that term. terry It is the RPM where the rotating device reaches a natural harmonic frequency and starts to oscillate. This can cause vibration or the shaft can actually flex and bend. 3 1 Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533626 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 Best is I understand, it is a speed at which the shaft starts to get a standing wave set up in it. If you run at that speed for long, the shaft will start to "whip" and deform. Eventually it will fail catastrophically. Depending on speed it can fail in a very short time period. 2 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533627 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 It is why I cringe when I hear someone say they are taking an old truck and adding O/D aux and an O/D main to gain road speed. The reason they aren't spitting driveshafts left and right, is the shorter the shaft the higher the critical speed, but it can't be "assumed" that just because the added O/D shortened the shaft you are not in critical territory. As my problem seams to indicate, totally stock might even be a problem. I have the factory manuals and they spell out the size and length of the shafts the truck came with, plug those into Spicer's calculator and one can see there is a problem! 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533628 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 My Marmon with 2spd rears sets up a vibration up against the gov in low range top gear, I haven't plugged in the numbers but I suspect that puts the shaft into or near critical. It has a Eaton O/D 15 speed, I never run it there, when I was climbing a hill and nearing where it vibrated, I would upshift the rears and down the main and carry on, so shaft speed is decreased. If I get bored I'll have to measure and plug in the numbers, but I bet that is what it is, feels like it anyway. Only shows in top gear, so any engine speed in lower gears doesn't vibrate, has to be driveshaft speed related. I never ran the truck up against the gov in high range on the rear axles. Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533629 Share on other sites More sharing options...
doubleclutchinweasel Posted June 26, 2023 Share Posted June 26, 2023 Yeah, spinning an old driveshaft at modern speeds can be a disaster waiting to happen. Do you suppose they never planned on those old trucks going that fast? Or do you think the numbers were less conservative back then? Or do you think they just had no clue? 1 Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533630 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 My guess is they didn't have a clue. The reason being that the truck I'm working on with the small tires I believe it came with (7.50x 20) would put driveshaft speeds at or near critical at around 50-55. With 8.25x 20 tires the max speed is around 60 MPH with the engine at 3000 rpm, max recommended speed. Even if you figure 45 MPH as max back in the day, it would be in dangerous territory, with the smaller rubber. My other K-7 sits on 9.00x 20's and that would put the max speed near 70, something it cannot reach. empty weight is 10,500 lbs 269 CID isn't going to move it that fast.That one I can see why it wasn't a concern 3000 rpm x O/D ratio of .823 puts the driveshaft at around 3650 rpm. Because the spec's for tire size are so far removed from driveshaft spec's I don't think they would have tied them in some way ("your can't order that wheelbase, with that tires size") I think they just were not well versed on the problems associated with shafting at that point. I could be wrong, but that is my take on it. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533632 Share on other sites More sharing options...
doubleclutchinweasel Posted June 26, 2023 Share Posted June 26, 2023 17 minutes ago, Geoff Weeks said: My guess is they didn't have a clue. The reason being that the truck I'm working on with the small tires I believe it came with (7.50x 20) would put driveshaft speeds at or near critical at around 50-55. With 8.25x 20 tires the max speed is around 60 MPH with the engine at 3000 rpm, max recommended speed. Even if you figure 45 MPH as max back in the day, it would be in dangerous territory, with the smaller rubber. My other K-7 sits on 9.00x 20's and that would put the max speed near 70, something it cannot reach. empty weight is 10,500 lbs 269 CID isn't going to move it that fast.That one I can see why it wasn't a concern 3000 rpm x O/D ratio of .823 puts the driveshaft at around 3650 rpm. Because the spec's for tire size are so far removed from driveshaft spec's I don't think they would have tied them in some way ("your can't order that wheelbase, with that tires size") I think they just were not well versed on the problems associated with shafting at that point. I could be wrong, but that is my take on it. No, I'd bet you are right on the money on that one. Ah, the good old "innocent" days! 1 Quote "Eagles may soar, but weasels don't get sucked into jet engines." Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533633 Share on other sites More sharing options...
Vladislav Posted June 26, 2023 Share Posted June 26, 2023 Pretty interesting subject to discuss, thanks for lifting it up. Honestly I don't remember hearing about those crytical speeds in the past but the effect sure makes sence. My guess older day's engeneers were familiar with that. Otherwise they wouldn't design shafts with intermediate carrier at all. Probably the road speeds were less in those days, especially for trucks. Vlad 1 Quote Никогда не бывает слишком много грузовиков! leversole 11.2012 Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533636 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 1 hour ago, Vladislav said: Pretty interesting subject to discuss, thanks for lifting it up. Honestly I don't remember hearing about those crytical speeds in the past but the effect sure makes sence. My guess older day's engeneers were familiar with that. Otherwise they wouldn't design shafts with intermediate carrier at all. Probably the road speeds were less in those days, especially for trucks. Vlad That is a valid point, they did put carrier bearings (both trucks have them) . They may have done "seat of the pants" engineering, however. If it didn't vibrate too bad it was good to go, now we know how and when the harmonics will kick in. I am itching to see how it runs out when the new shaft get here. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533639 Share on other sites More sharing options...
Popular Post Joey Mack Posted June 26, 2023 Popular Post Share Posted June 26, 2023 (edited) G.W. I see you said you have a K-7 on 900/20's.. I would love to see some pictures... I have A '48 KB-7.. I'm quite proud of it.. I will go ahead and bore the other guys with pic's again.. Edited June 26, 2023 by Joey Mack Pour sperring 4 2 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533640 Share on other sites More sharing options...
Popular Post Geoff Weeks Posted June 26, 2023 Author Popular Post Share Posted June 26, 2023 (edited) Mine are still in their work clothes. what size rims do you have on yours? I've got 6.5 (edit 6 inch on the 8.25, 6.5 with 900's) with 8.25x 20 rubber and they are stamped max tire size 8.25. The one with 9.00 had disk wheels. Edited June 26, 2023 by Geoff Weeks 3 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533642 Share on other sites More sharing options...
Joey Mack Posted June 26, 2023 Share Posted June 26, 2023 9.00/20's Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533643 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 what is the rim width? I am wondering how well they would fit on the narrow (er) rim. 9.00's normally go on a 6.5 or 7" rim. As I said my rims are stamped with the max tire size My truck with disk wheels has 6.5" 6 bolt wheels, the spoke are only 6", which is why they are stamped with the 8.25 tire size. Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533644 Share on other sites More sharing options...
Joey Mack Posted June 26, 2023 Share Posted June 26, 2023 I dont remember right now.. If I get a chance later or tomorrow, i will look.. I just painted them, so not sure if I can read them. I can go ahead and run a tape measure across.. Give me a minute,, Im gonna look now instead.. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533645 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 They look wider than mine, do you have any clearance problems? I once tried some wider rims and the inside rear was close to the springs, and the front to the tie rod end. Nice you have the hubcaps. they often go missing. The battery box cover: I don't think the trucks ever came with them, I have yet to see a truck with one on it. I had mine made. Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533646 Share on other sites More sharing options...
Joey Mack Posted June 26, 2023 Share Posted June 26, 2023 OK, the rims seem to be 8'' wide from edge to edge... I have a battery cover for it.. this is a project truck and I have many parts in my shop.. I need a starter for it. Im going to change it to 12v negative ground.. it runs good.. I also dont have a crank handle to start the engine either.. Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533649 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 Those are 6.5 or 7 (it is the inside width between the rim flanges" I can tell exactly.. Wow you got the "optional cover!" every one I see it is missing. Re Starter, is yours missing? they are easy to rebuild and aftermarket part (armature etc) are not expensive. They have a oil cup on the brush end of the commutator that needs to be lubed often as there is no brass bushing in that end ( steel shaft on cast cover). Mine start un assisted at 0 deg F on six volt, so I am keeping them at that voltage. That way the gauges and wipers will not be effected nor need special adaptation. I have a crank that fits the engine but was not for the truck and needs to be longer to use. Both mine have 2 spd rears and 5 O/D trans, but they could have 4 spd and single rears, or any combo. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533650 Share on other sites More sharing options...
Geoff Weeks Posted June 26, 2023 Author Share Posted June 26, 2023 If you need a complete starter it is Delco number 737Z 1 Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533651 Share on other sites More sharing options...
Joey Mack Posted June 26, 2023 Share Posted June 26, 2023 mine is 5 spd single rear, my starter has a hole in it.. i could rebuild it and weld a patch over the hole.. Im only trying to make it road worthy for local driving and shows.. Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533652 Share on other sites More sharing options...
Joey Mack Posted June 26, 2023 Share Posted June 26, 2023 Thank you for the starter part number... I spent over an hour trying to find one.. Quote Link to comment https://www.bigmacktrucks.com/topic/72980-u-joint-critical-speed/#findComment-533653 Share on other sites More sharing options...
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