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Thread: Help with draw Bar opinions
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30th Sep 2012, 02:59 AM #16
My opinion is that it will give you a stiff 125 x 50 drawbar, due to the extra thick webbing in the middle. Not overly stiff however. Still good enough for the job. It may be worth tying the ends of the 50 x 50 to the main frame and hitch plate with a plate along the sides. Just a thought tho.
Dean
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1st Oct 2012, 12:23 AM #17Intermediate Member
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Stiffening the drawbar
From a structural engineering perspective, a drawbar is essentially a beam with a point load partway along its length It is loaded at the point where it attaches to the front of the chassis. Maximum bending stresses are at this point (where the drawbar is attached to the front of the chassis). Maximum bending deflection will be a little in front of this position, depending on the length of the drawbar.
It would be important to run any stiffening section almost the full length of the drawbar, particularly past the point of attachment of the drawbar, back to its rear near the spring shackles. A 50 x 50 x 3.0 section stitch welded under the existing 75 x 50 x 3.0 (I don't think there is a 3.5), would give a combined section better than a 125 x 50 x 3.0
Styx
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1st Oct 2012, 09:29 AM #18
Yep. I can confirm the statement " Maximum bending deflection will be a little in front of this position, depending on the length of the drawbar" with personal experience.
This Method sounds like the go then.
I hope the OP is reading this and not out adding his truss as we ponder the best method to fix his problem.
Dean
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1st Oct 2012, 11:00 AM #19
Definitely reading dean.... Have not done the truss over the weeked after the latest developments
Might have to take this job into a fabricator though as i dont think my small mig will be up to the job of laying a nice big weld between the rhs radius to join them together safely
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1st Oct 2012, 04:05 PM #20
Sorry, I wasn't on over the weekend, but as it's already confirmed by Oldneweng & Styx that this is a better way for you to go, the only other thing I can add is that the reason that adding the extra 50 x 50 x 3 underneath will actually be slightly better is that as a single section 125 x 50 flexes, it will bow out the sides of the 125mm section, whereas the two 'laminated' lengths effectively have a rib down the middle.
And just to expand on Styx's comment, if you only add a short section to strengthen a weaker member, then all you do is change where the weak point is.Too many projects, so little time, even less money!Are you a registered member? Why not? click here to register. It's free and only takes 37 seconds! Doing work around the home? Wander over to our sister site, Renovate Forum, for all your renovation queries.
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1st Oct 2012, 06:46 PM #21Intermediate Member
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Stiffening the drawbar
At the risk of over-simplifying the drawbar problem, it is feasible to model the drawbar as a simple beam with a point load part way along its length.
Bending and deflection is OK until the bending stresses exceed the elastic limit of the steel, and then you get a permanent deflection, i.e. a bend. It is important to understand that usually permanent deformation is due to excessive bending stress. (Local buckling can also cause permanent deformations, but we assume we are not at buckling loads with RHS members.)
Assuming the simplified arrangement is as shown in the attached diagram, it can be seen that the maximum bending stress occurs at the point B where the front of the chassis loads the drawbar. The stresses reduce to zero at each end of the drawbar. (It can also be seen that the longer the drawbar, the greater will be the maximum bending (moment) stresses.)
To strengthen the drawbar by adding another section underneath, it would be both practical and structurally more sound to not have a sudden change in strength along its length, which would happen if the strengthening did not run the full length. 50 x 50 x 3.0 RHS is about 4.25 kg/m, so an extra 6 m would add 25.5 kg, which is about the weight of a spare wheel.
The depth of any truss type strengthening should be maximum at B, the front chassis point, and should continue in both directions before tapering to zero at each end.
Hope this is useful.drawbar.JPG
Styx
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1st Oct 2012, 07:41 PM #22
Hi yonnee
Thankyou for your comments i have read them carefully a i completely understand what you are saying and im pleased that the extra 50x50x3 will be as strong or stronger then 125x50 .... Ill run the new 50x50 the entire length of the drawbar from tip to end at leaf mount ....the third draw bar would fail at the moment it stops simply transferring te problem somewhere else
I want it to be very strong as it carries a 1400kg cv8 monaro track car
Hi Styx
thanks very much for your comments also .... I would prefer not to go the truss idea as it would be sooooo large due to the drawbar being over 3m's from hitch to leaf mount
i agree that the edge of the deck on the drawbar would be the first point of failure and any attempt to fix by not running the whole length would only move the issue to another area.... So running rhs under the whole length of both sides seems a great fix to me ... The extra weight will be worth it and im shedding heaps getting rid of the ramps and going to aluminium planks
As i said now im just worried my 150a mig wont be able to lay down the big weld needed to join the 2 rhs on the radius
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1st Oct 2012, 09:41 PM #23Diamond Member
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1st Oct 2012, 10:40 PM #24Intermediate Member
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Stiffening the drawbar
The welds are important, and from my rusty recollection, the strength required depends on the vertical shear at any section.
From the simplified model I suggested, the vertical shear is higher between the rear of the drawbar (C) and the front of the chassis (B).
In fact it is likely to be double the value between the coupling and the chassis. As the calculations are quite complex I suggest a practical approach:
1. Remove the paint from the lower edges of the drawbar for the entire length.
2. Obtain the 50 x 50 x 3.0 RHS, cut to length, clamp underneath, and use the 150A MIG to tack weld along the length both sides.
3. Take the trailer to a professional welder/steel fabricator and get them to weld the section as they think fit.
Styx
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1st Oct 2012, 11:21 PM #25
Hi styx
Exactly the plan i had in mind ill put it together here then tow it to a fabricator with the knowledge and equipment to lay some nice stitch welds down inside and outside ..... a stitch on the outside then a gap during the gap lay a stitch on the opposing side
Hi rustyarc
Yeh i suppose thats true but might be a little beyond me and i want to make sure they are joined very well
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2nd Oct 2012, 08:48 AM #26
If you look at the stresses created by a load on the trailer the added section will be under bending/stretching stress. If enough weld is used (as suggested in a previous post) and the welds are properly done I would think this would be the end of your problem. I am not an expert tho.
I bent my trailers drawbar many tears ago overloading it with firewood. It is made of 50mm X 6mm angle. I boxed it in with another piece of same each side, stitch welded it and never had another problem. It often gets overloaded. I carried 2 slabs of concrete 150mm X 1200mm X 2100mm twice earlier this year. It is only a single axle box trailer. The slabs had to sit on pallets above the sides.
A truss provides great strength but a simple one like you proposed is only intended for static loads. There are issues with fatigue at the vertcals mounting points as shown by the link to the camper. Using the method we are suggesting is far more capable of handling non-static loads.
Again I am not an expert and my knowledge is based on other peoples ideas, my experience and logical thinking.
I would have thought that your 150A mig would be capable of this job. if you are not convinced do a small weld, break it and check. I just googled "check weld depth". Have a look at that. Some PDF files showing methods to use to check your welds.
Dean
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2nd Oct 2012, 10:27 AM #27
Hi dean
Thanks for the comments sounds good ill order the steel today .... Shouldnt be too expensive and i think the end result should look ok too
Regarding the welds, yeh i think maybe my welder might be up for it but im thinking my experience isnt ..... I'll do research on the penetration tests and have a crack on some spare steel i have here first .... Id hate for them to rip off in a year from the 3 hr trips and constant flexing
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2nd Oct 2012, 11:04 PM #28
I reckon your 150A should be good enough. I can confidently do 6mm material with my 175A Mig. The difficult part will be welding on the side, as you can't really let the weld pool.
Too many projects, so little time, even less money!Are you a registered member? Why not? click here to register. It's free and only takes 37 seconds! Doing work around the home? Wander over to our sister site, Renovate Forum, for all your renovation queries.
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3rd Oct 2012, 11:05 AM #29
I can tilt it on its side to let the pool sit still ...
Ill give it a go when the steal turns up....
Ps pics to complete the mods will be put up soon
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3rd Oct 2012, 05:12 PM #30
OK then.
The best way to get the penetration you need is to run the weld in a triangular pattern.
The best way I can explain it is with a clock face. Start at 6 o'clock, weld through to 12 with a weld about 10mm long. Then without stopping, continue in an arc back towards the middle and out to 8 o'clock. Then accross in an arc to 4 o'clock. From there another arc back to 12 o'clock running 5-10mm in front of the first forward run. Keep repeating this in a continuous weld, 12 to 8 to 4 to 12 to 8 to 4, with each run to 12 running a bit further forward each time. Each of these welds should be about 40 - 50mm long.Too many projects, so little time, even less money!Are you a registered member? Why not? click here to register. It's free and only takes 37 seconds! Doing work around the home? Wander over to our sister site, Renovate Forum, for all your renovation queries.