It carry the crosshead guides and support the engine entablature (the cylinder block). On large engines, the A frames are individually erected on the bedplate directly above the transverse girders. When boxed in with plating they
form the crankcase. The trend nowadays is to build the frame box as a separate fabricated construction and then, after stress relieving and machining the mating surfaces, to mount it on the bedplate. This has the advantage of saving weight