Side force on trailer 2

[mrMikee]

I am working on a structure that folds down into a towing position for transportation to another site. It has a self contained triaxle arrangement that is heavily loaded, and when it goes around corners I am assuming that the front and rear axle will need to slide sideways to some degree as necessary to turn a radius. A search I did for the coefficient of friction of rubber on concrete indicated the max to be about 80% to 100%. I used 100% for the design but my structural analysis is indicating stresses in the cross members supporting the triaxle to be about 170 to 180% of allowable. The substructure I am using has been used before although it is relatively new and I have not personally observed it in operation.

Is 100% sliding friction a good assumption for side force? Is there some good information I should be looking at? Are there any helpful hints?

Thanks,
-Mike

 

[GregLocock]

Yes, 100% is fine, for mu.

However, in practice if you design to 1g your design will fail. The most likely large sideways force is when it hits the kerb. This can happen in two ways (a) the trailer skids out due to low friction/excessive speed around a corner (b) the driver forgets to allow enough room for the trailer so rides up over the kerb.

I would allow at least twice what you have (ie 2g loading) laterally if you wish it to have a long life. If either of the above manoeuvres is likely to happen often then a more detailed calculation may be justified.

Vertically you need to allow for 3-5 g (potholes), and longitudinally 2-3 g (potholes).

This is for normal on-road use.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mrMikee]

Greg,

Thanks for the information. The machine I am working on will move infrequently if at all after the first installation, so long life on the road is probably not a concern. But I am concerned about the (2g) lateral loads you mention because of conditions encountered with this machine when pulling off road into a jobsite.

Actually the vertical and longitudinal could be a problem too. I am using A36 steel (Fy=36 ksi & Fu=58 ksi) and designing to AISC (building construction) allowable stress levels. At the peak g forces you are talking about what should I be using for stress? I don't want to design too close but the reason I am involved in the redesign is because the previous machine was found to be too heavy.

Thanks,
-Mike

 

[ivymike]

sounds like you need bigger tractors, or much lower travel speeds.

 

[ivymike]

does it have to be 'self-trailering' if it's infrequently moved? why not rent a big (proper) trailer once in a while, and leave the wheels off it the rest of the time?

 

[mrMikee]

In the past I have put signs on the equipment to limit highway speeds, but unless you are traveling with the driver you have no idea what really happens.

-Mike

 

[mrMikee]

This machine is big. It's over 60 feet long and weighs more than 60,000#. I preferred going with a drop off axle concept too but there were reasons at the time not to do this. Instead it was decided to do a redesign and take out excess weight.

-Mike

 

[MikeHalloran]

You have to assume that it will travel on the highway at 120mph.

Time is money.





Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

That's a pretty robust steel.

However, we'd normally use a much better steel than that, say extraform 400, and then I'd design to yield at 2g lateral, for a first look. In terms of cost/stress the good steels usually come out at least lineball, if not better, which gives a nice weightsave.

Chassis sections are often of quite a fancy steel, Possibly not justified for you usage.








Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ccw]

Hi mrMikee,

You did not specify if this was a semi-trailer, but I assume it is. So, is the tri-axle arrangment clustered close together or spaced out under the frame? Have you set the tri-axles up to give 30%-50% down force reaction on the tractor fifth wheel? (You will need this for off-road traction) These considerations will affect the lateral skid loads and stresses when cornering.

 

[mrMikee]

Thanks all for the comments.

Here's a little more info. The triaxle group is 9' long (2@4'6") and carries about 65% of the trailer load leaving about 35% on the fifth wheel. This is primarily a structural product built with common structural shapes, so anything exotic material wise is not an option. I will be going to more grade 50 steel however.

Regards,
-Mike

 

[BillPSU]

I would recommend you go up to a 80,000 yield with high formability to reduce your wieght. It is still weldable without preheat and will reduce your overall weight.

Will the tandems be adjustable to move the CG around or will this be fixed?

Another problem will be where the tandems are located. With the trailer being over length, turning this machine will be difficult. I hope this machine is not going to be used in urban settings as making turns within a city will be difficult.

 

[aggman]

Mike,
For similar portable equipment we have always designed everything for 2 to 1 impact with standard structural shapes or plate girders as required. We account for emergency stop (compression in the frame) utilizing vehicle tractive forces. We also always make sure that you have a "good" diaghram built into the chassis to handle lateral loads. The odds of a trucker pulling a semi-portable chassis 120 mph is completely unrealistic. Most of these things are over weight, over width, and over height. They are lucky to get them to the speed limit. The axle frame is generally robust already due to the heavy weight. We just make sure that we use sound force transfer in the frame and the lateral loads will generally not be a problem. Also note that when the customer is going to move a high dollar portable plant down the road they will not be doing it when its snowing or in other incliment weather due to the risk. So while everyone here has given you good advice with respact to automobiles and trailers, it is not real useful for your application.

 

[mrMikee]

The location of the triaxle is fixed. Moving it back would have solved my axle load problems but would have created others concerning erection and functionality of the equipment. Also it would be less maneuverable which is already an issue. In my redesign I have been able to take out some weight with more efficient framing and attention given to work points, but have added almost as much back in for deficiencies in other areas.

Oh well, I guess I'll up the steel grade and work on this some more.

-Mike

 

[MikeHalloran]

Maybe 120mph is unrealistic, but 90 is not.

Predawn, somewhere in coastal SC, I pulled onto I95 SB, behind a couple of 18-wheelers. I stayed behind them, as is my habit. After a few minutes, my station wagon started getting a little twitchy. I looked down at the speedometer.

We were going 95 mph, and the trucks were still accelerating.

All the while, the drivers were chatting about their heavy loads, and how much they loved their Caterpillar engines. The MEDIUM power versions.

I backed off to ludicrous speed.

I've seen lots of oversize, odd stuff on that road. None of it was going slow.



Mike Halloran
Pembroke Pines, FL, USA