The only chain driven Trikes I have seen are of three types:-
1. Rigid frame - the axle stays rigid in the frame.
2. Swinging arm suspension - the axle is mounted on a very strong swinging arm so that it can ONLY move vertically, resisting any body roll so that no twist is imparted to the chain.
3. Independent suspension - the differential is mounted on the chassis and the wheels allowed to move in any direction they like. The diff maintains it's relationship to the chain.
Any twist on a chain will wreck it instantly. Body roll is not desirable on a Trike anyway, it feels awful.
The axle I used was from a Reliant Regal, you can tell this axle as it has nuts on both sides of the diff casing bolting the whole thing together, the Robin/Rialto axle only has nuts on one side. I've never been inside a Robin axle so I can't help much there. I had to shorten one side of the Regal axle for my mkII Trike. These instructions are based on how I went about it.
Decide which end you are going to shorten it to align the chain and by how much. Remove the brakedrum, backplate and halfshaft, Scribe a line along the axle and acrosss the end casting so it will all line up later. By removing the 2 holding bolts and heating the casting (I only used propane so it wasn't that hot) I was able to pull the casting from the end of the axle. Saw off the required length, machine (or use a sanding disc like I did) the axle tube back to the correct diameter so it's suitable for shrink fitting the casting, then the casting can be re-heated and re-fitted to the shortened axle tube. (you did line up that scribed line didn't you?) Drill through the bolt holes and re-tap (5/16 UNF). that's the outer tube done. If the end casting comes loose you can tack weld it to stop it fretting (well it's held on mine).
The halfshaft takes a bit more work. I'll work on the assumption it'll be shortened by 50mm. Have a look at the picture below.
On a Regal diff the crownwheel is rivetted on, grind the back off the rivets and punch them out. I had a new sprocket made by
Talon Engineering in Yeovil, Somerset with a hole in the middle to fit the diff and 8 holes to bolt it on (they also made a new front sprocket
to keep the gearing right).
Since writing this originally another (simpler) way has been developed The axle tube is cut at the position of a mounting point,
another tube is fitted into the axle tube and the whole assembly welded back together. The joint is then strengthened by the fact that the mount is welded across the joint - see below.