Truck trailer statics

[geopat69]

Hi

Im trying to understand the statics of how this possibly works.

5 axle truck trailer 30m long overall on the show room floor. Self weight of trailer is 25 tonnes.

All 5 axle rows (1.8m apart) are hydraulically linked so the centre of reaction is theoretically under the third axle. Trailer has a king pin but is not connected to a prime mover yet.

The drawings say the centre of gravity is 1.5m forward of the 3rd axle. [So.theres effectively an eccentricity between centre of reaction and centre of gravity of the trailer).

If all axles are hydraulically linked...how is it that the trailer is not tipping forward?

 

[271828]

I think a sketch might help.

 

[JStephen]

My guess- the axles are not hydraulically linked, or one or more are at the limits of their travel.

Your question reminds me of a situation long ago. The company had a 4-wheel rough-terrain crane loaded on a lowboy trailer. One of the workers there was convinced that if they put the hook of that crane on the front of the trailer, the crane could then lift the trailer off the truck. And he could not be persuaded otherwise. Fortunately, he was not in a position to attempt it himself.

 

[3DDave]

Sounds like a trailer I worked with - the hydraulic suspensions of the front two axles were tied together and the rear three were tied together. It also had a hydraulic gooseneck plumbed to transfer the front axle loads to the fifth wheel as well as allowing different fifth wheel heights while keeping the trailer level.

 

[GregLocock]

Are there any springs in addition to the hydraulics? what is the 'wiring' diagram of the hydraulics? Another possibility is that the piston size or motion ratio of the rams is different on each axle.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[geopat69]

Hi. Reconfirmed with the manufacturer.

All hydraulic lines are definitely linked. And the offset of 1.5m forward is definitley the case.

Strange. Googling these types of trailers, i see a lot of photos (unconnected to prime movers) being similar and not tipping forward. And i suspect all would have a COG forward from the centre of reaction of the trailer axles.

I must be missing something but if the hyraulics are equalised on all 5 axles (its like an air mattress under the deck), the centre reaction is precisely at the third axle...surely the trailer should be tipping forward.

I do understand that if the hydraulic is... say a 2:3 split...then the hydraulics of one line could be higher and create a statically determinate situation.

Wierd and dont know what im missing?

 

[BridgeSmith]

There has to be some type of control mechanism, either hydraulic (control valves) or mechanical (springs?). Otherwise, any imbalance would cause it to tilt, and keep on tilting, unless some of the hydraulic cylinders are different diameters, in order to create different forces with the same pressure. Even then, if the COG changed, it would tilt. Even with springs, it would tilt some, unless it had larger springs under the heavier end.

Forgive my ignorance, but what is the purpose of this trailer?

 

[3DDave]

Care to share exactly the trailer and schematic?

 

[rb1957]

forgive my ignorance, but don't hydraulic systems level the vehicle ? Your assumption is that the sum of the five axles is over the middle axle, assumes all 5 have the same load.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

The axles that are plumbed together do distribute the load evenly. There should be a separate hydraulic pump and reservoir to set the ride height to match the kingpin. This does require the trailer be stiff enough to handle the even loading on the suspension side and the possibly concentrated load on top.

It makes for a far smaller load on the road at steps and bumps. As the initial tire starts to rise up, it pumps fluid to the trailing suspension causing the rest to rise up. A bump at one cylinder extends the other cylinders so the deck only goes up 1/nth of the group. This makes for a far smoother ride. I think the trailer I worked with could clear a 4 x 4 timber at 30 mph without trouble.

That's why I'm interested to see what the plumbing really is. Ours was a group of 2 and a group of 3.

 

[rb1957]

so the load on all 5 axles is not necessarily the same ?

I was thinking about the hydralastic suspension like on the old mini, which I thought kept the car level under shock loads (like uneven ground)) and so each actuator had a different load.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

The load on whatever sub-group is the same. Right side is separate from left side to keep the trailer from rolling over.
For exzmple

https://www.internationalcranes.med...mt-order-from-schaften-cranes/8023574.article

https://youtu.be/jNnxCJfStAc?t=108

 

[dhengr]

Geopat69:
A five axle trailer is a very large cap’y. over the road trailer (low boy). Usually loaded from the front, and always going to be slightly heavy on the front w.r.t. the five rear axles, in the lt. wt. configuration, that’s why it lays down on the ground, when you disconnect the goose neck for loading. After all, 60 or 70% of the real heave body structure is ahead of the rear axles. Some plans, elevations, pictures, schematics, manuf’er. name and lit. would b most helpful. There are a half dozen variations on the theme, and we can’t see them from here. Your trailer might have one or two flip axles on the rear and only three fixed axles on the trailer itself. The flip axle is hinged off the back of the trailer, hydaulicaly rotated down on to the pavement to cause about 2/5 ths. of the load to be taken by these axles. Some axles have air suspension too. Statically, the wheel loads are all equal, but depending on the reaction speed of the system and the speed of actual travel you will get some variation.

When you have one axle across the trailer, five axles sets in total, dual wheel sets on each end (one dual set on each side of the trailer, on each axle), they have to be controlled in two ways, by volume and by pressure at each axle. Two or three of the axles can also be lifted off the pavement, carried by the trailer structure during lt. travel. The hydraulic volume and its control relates mostly to pulling the axles over a hump or a valley (a curb or depression) in the line of travel, and the front, center and rear axles will have the most vert. travel, over the curve, in that process. For the most part, this sets the cylinder and reservoir volume for each axle, some iteration, engineering experience, and kinematic software involved here too. Now, if we have enough cylinder volume for the max. vert. travel we want, then we have a pressure control at each axle too, which allows the vert. dual movement, side to side, to account for the cross slope variations. Mostly, this involves no volume change, and constant pressure on both sets of duals, for equal load on each set of duals on an axle. Thirdly, the axles are all interconnected for pressure control so they share the load equally, and this will change the volume to each axle to cause this load ratioing or balancing axle to axle. That’s the general scheme of things, and the exact plumbing and wiring and pressure switches, check valves and pressure relief valves will vary from one manuf’er. to another.

There are also heavier, more specialized trailers (load platforms) which can be connected side by side and end to end to make up a bigger size (footprint) and load unit. On these, each dual set pivots and is individually hydraulically controlled for pressure, volume and steering. You can turn these on a ‘large dime’ without dragging 40 sets of wheels around the turn, and on a much tighter radius, since the trailer unit steers.