My workshop, like yours, could do with more space. I am at or beyond the point where if I want anything new I first have to work out whether I can fit it in. To make things more manageable, I recently fitted mobile bases to some equipment, including to some big racks I built for materials and part-finished work. These items now live in the corners and get pulled out when needed. I have clear workspace in the middle. Luxury!
The first of my mobile bases was a commercial one for my table saw. It worked well enough to encourage me to go further. The second and third were pairs of rollers of the kind used for kitchen appliances, which I put under the bandsaw and the router table. They were much less successful as they don’t steer easily and tend to get out of place. Also, their very small wheels tend to get stuck when there are shavings on the floor. I plan to replace them.
My next move was to get some heavy-duty casters and build my own mobile bases. They are much less expensive than the commercial bases, just as useful, if not more so, and better than the appliance rollers. The simplest construction is a square of plywood for the item to stand on, with a caster under each corner. But suitable casters are several inches in height and may make tools too high for easy use, and less stable.
A different construction method can keep the equipment close to its original height, at the cost of putting the wheels outside the item’s footprint. This is not necessarily a problem, and it does make the item more stable. The casters fit under the ends of a pair of cross beams, either timber or angle iron. The beams go next to the unit, not under it. The item stands on a plywood square. But the plywood is wide enough to hang from the underside of the cross beams, between the casters. Spacer blocks between the beam and the plywood drop the plywood platform close to the floor. The increased overall height is then just slightly more than the thickness of the plywood.
I found that swivel casters on all corners are the easiest to use for mobile bases as they let the item turn on the spot. The casters need enough clearance to swivel without hitting anything. Larger wheels roll more easily if the floor is uneven or has shavings on it. I find they don’t need brakes, unless perhaps if the floor is super-smooth and clean, or sloping. Wedges slipped underneath the item stop it moving. If a mobile workbench or machine will need to resist sideways forces, you will have to stabilise it. You would need to wedge it carefully to lift the wheels clear of the floor. Brakes on the wheels would have to act on the swivel as well as the wheel itself. So I suspect they would be less effective than the wedges.
There are plans available for trolleys that lift the item for moving and let it down again when in place. These would be best for stability and convenience, but are more complicated to build.
I tried casters with a single fixing bolt and the kind with a steel plate that you fix to the beam with screws or bolts. I think the plate fixing is easiest for fixing to plywood or timber.