You have a 6 axle outfit, 3 axle truck, 3 axle trailer.
It has a maximum GVW of 44 tons, but because you have heavy duty axles your vehicle has much less chance of falling foul of axle weight issues than many**
Steer axle rated at 9 tons, it would be almost impossible for you to exceed this.
Stree axle weights vary very little on artics between loaded and empty, on my 6x2 mid lift, it weighs only 200/400kg different, your steer axle assuming you lift the rear tag when empty will have more difference due to fifth wheel position, but for all practical purposes you can foregt the steer axle weights because you can’t do much about it whatever you do, you have plenty of leeway be assured.
Drive axle itself is rated at 11.5 tons, in practice if you were to use an axle weigher you will find that when loaded the drive axle will gross at somewhere around the 10 ton mark.
Rear axle is a tag, will usually be liftable when unladen, typically that axle will weigh anywhere from 5 tons upwards if fully loaded.
By the way i’m impressed just how light your tractor unit is given the heavy duty axle specs, would have expected it to be over 8 tons.
The trailer axles as with almost all 3 axled trailers all axles are 8 tons max, when on air suspension unless there’s a design or suspension fault the weights imposed on those 3 axles won’t vary much more than 500kgs maximum and usually within 100kgs of each other.
You say you have a tare weight of 18tons, and the tractor weighs 7.5, that’s a heavy trailer in itself.
Assuming its been designed well, an evenly spread load would probably see your actual loaded axle weights reading something like, 7 10 6 tractor, 23tons gross and 7 7 7 trailer 21 tons gross, this would be safely and legally loaded vehicle that would probably be extremely stable on the road, i aim for 7 to 7.3 tons on my trailer axles when fully loaded for stability legality and suitable drive axle grip especially in slippery conditions.
You can theoritically shift the load forward on your outfit, up to the maximum GVW of the tractor unit, but beware there are also maximum king pin loads to consider, if i recall correctly these will be found on the trailer plate, not as i’ve ever known VOSA or whoever they are this week to be weighing the imposed weight on the kingpin of a trailer.
** in comparison, a typical 6x2 tractor specified for max weight work might well be fitted with a light mid lift axle on small wheels for obvious weight saving.
The max weights on such a tractor could be in the lines of 7t, 5.3t, 11t, as you can see this doesn’t give anywhere near as much leeway for load distribution as your heavy duty design rear tag unit has, and i have seen those mid lifts rated as low as 4.6t, very very easy to fall foul of axle weights when the total combination only just exceeds the 44t max allowable, much playing around with fifth wheel positions and load positioning to find the sweet spot, not as most drivers care about such things and from what i observe nor do a number of operators.
If you can try and use an axle weigher, both loaded and empty and make a note of the weights so you can peruse at your leisure comparing them with the plate weights.
If you can’t find an axle weigher but have the use of a flat weighbridge, ie not one up on a ramp, and the operator is friendly you can work out all your axle weights, surprisngly accurately too.
Drive on with 1 axle, weigh it, two axles, weight it, three axles, weight it, then the whole vehicle, weigh it…you don’t need to individually weight trailer axles because they will all be within 200/300kg of each other,. but you can of course weigh the trailer axles alone for confirmation.
Gross weight minus trailer axles gives you the two split weighs.
Get pen and paper and do your sums.
As Franglais (much clearer than my waffle 
), between us hopefully we’ve shone some light.
