Outwardly, they *are* very similar – they both are powered by jointed driveshafts, driven by a gearbox from a return crank on the main driver crankpin. The main differences are inside the cambox mounted above the power cylinders. Both mechanisms are hard to visualize without referencing drawings, but a basic comparison follows:
In the Franklin Type B, the valve timing is set by a continuous contour camshaft. Valve opening and closing events are determined by the profile of the cam, which varies continuously along its length. To adjust the cutoff, or switch from forward to reverse, the camshaft slides laterally via the reversing mechanism, thereby presenting a section of the cam profile to the cam follower on the valve stem. The cam follower is a spherical bearing that makes point contact with the camshaft. The valve timing is controlled by the shape of the camshaft where the follower makes contact. In the Franklin system, the cam follower acts in direct line of action on the valve stem, which is oriented horizontally, parallel to the piston rod. Valves are closed by coil springs acting on the valve stem.
In the Caprotti gear (British Caprotti specifically) a variable geometry cam assembly is used. Essentially, there are two very similar cam lobes for each valve, each on separate concentric shafts, which can rotate relative to each other. To adjust the cutoff, or switch from forward to reverse, the angle between the two adjacent cam lobes is altered via a worm gear and crank assembly. The cam follower is a cylindrical roller that makes line contact with both cam lobes simultaneously. The valve timing is controlled by the combined shape of the two cam lobes where the follower makes contact. In the Caprotti system, the cam follower acts via a bell crank on the valve stem, which is oriented vertically, perpendicular to the piston rod. Valves are closed by steam pressure acting on the valve stem.
In both cases, the camshafts are mounted parallel to the driver axles, and operate hollow, double seat poppet valves. When the valves are open, steam passes around and through the body of the valve. There are detail differences in the shape of the valves between the two systems, but they are generally similar in design.
The rotary cam T1 uses a derivative of the Franklin Type B system, called the B2. Unlike the Caprotti system, which uses one cam assembly to operate both intake and exhaust valves, the Franklin B2 has two camshafts – one exclusively for intake valves, and a second one just for exhaust valves. Also, the B2 has 4 valves (2 intake and 2 exhaust) where the Caprotti has 2 (1 intake and 1 exhaust) at each end of the cylinder.