Now I have improved my linear scanner.
It's also low-cost and all components are coming from the home-improvement market.
There is no limit now of the travel. The whole length of the 2m long aluminium rail can be used for scanning. That means it can be used as a full-sized scanner for persons.
The sleigh consists of an aluminium base plate with four wheels which normally will be used for ropes. They are made of polyamide and are smooth-running. The rail on which they are moving, is a 40mm wide and 6mm thick anodised aluminium profile. It is mounted with a small gap onto a 25mm x 25mm aluminium carrier. This space between the two aluminium parts is necessary for the returning cord.
Inside the nave of the wheels are brass sleeves to enable a precise and easy rotation of the wheels. These sleeves are clamped by the screws. The holes in the base plate are a bit bigger than the diameter of the screws to enable a fine adjustment for a perfect run.
Edit: Because these kind of pulleys are normally fabricated by injection moulding take care of burr on the running surface. You can carefully remove it with a fine round file.