Cam drum
The cam drum , also known as a cam disk or cam ring, is used in valve-controlled four-stroke radial engines to control the valves and corresponds to the camshaft in a conventional reciprocating piston engine.
It is a ring-shaped or disk-shaped component that is arranged concentrically to the crankshaft of the engine in the engine housing and is driven by the crankshaft via a reduction gear, whereby it can rotate in the same or in the opposite direction as the crankshaft, depending on the construction . On its outer circumference it carries the cam tracks with the actuating cams for the valves, which are arranged in two planes one behind the other, one for the inlet valves, the other for the outlet valves. The transfer of the control movements to the valves is usually done by roller tappets, push rods and rocker arms running on the cam tracks . The cam drum can be arranged both in front of and behind the cylinders. With multiple radial engines, each star requires its own cam drum. This arrangement was used for the first time in rotary engines, in which usually only the exhaust valves were controlled via push rods, so that only one (fixed) ring with a single cam plane was required.
In contrast to a conventional camshaft, which usually runs at half the crankshaft speed and has a single cam lobe above the base circle for each valve, several cam lobes can be evenly spaced around the circumference of the cam drum. This requires different reduction ratios: the reduction of the cam drum is 1: (z + 1) when running in the same direction, where z is the number of cylinders, or 1: (z-1) if it rotates in the opposite direction. That means, for a radial engine like the Siemens Sh 14 with seven cylinders and a cam drum rotating in the same direction, it is 1: 8. In this case, the number of cams is four each for inlet and outlet control.
In the BMW 132 engine with nine cylinders and counter-rotating cam drum, the reduction according to the above formula is also 1: 8 and the number of cams is only four each.
The necessity of a cam drum results from the special feature that in the radial engine all cylinders are arranged in one plane, but at different angles radially around the crankcase and the axis of rotation of the crankshaft. The concentric cam drum enables, with a single component, largely straight controls between the cam tracks and the cylinder heads with the rocker arms and valves of the same length for all cylinders; Without a cam drum, a number of individual camshafts corresponding to the number of cylinders with the associated drive gear sets would have to be arranged around the crankshaft.
literature
- Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden 2005, ISBN 3-528-23933-6 .