Fig. 443.—General Electric type M P, marine generating set with tandem compound engine. The requirements of such units are compactness, light weight, simplicity, freedom from vibration and noise at high speed, perfect regulation and durability. By adopting a short stroke for the engines and a special armature winding for the dynamos, the height and length of the sets have been reduced. The bed is carried out to the full width of the dynamo frame, making an ample base surface for foundation without increasing the floor space required. While the construction gives a massive appearance, the bed has been cored out and the various parts so designed that the complete sets have an approximate capacity of 3½ watts per pound. All of the moving parts are enclosed by the engine column, excluding dust and reducing wear and attention to a minimum. The bearing are oiled automatically under pressure. These sets are made in sizes from 25 K. W. to 75 K. W., the cylinder dimensions for the smallest size being 6½ and 10½ by 5, and for the largest size 10½ and 18 by 8. Single cylinder sets are made in sizes ranging from 2½ K. W. to 50 K. W., the cylinder dimensions ranging from 3½ x 3 to 12 x 11. See fig. 730.
Ques. What is a direct drive?
Ans. One in which the driving member is connected direct to the driven member, without any interposed gearing.
[Fig. 443] shows a direct connected unit, which is an example of direct drive.
Ques. What may be said with respect to direct drive?
Ans. It is the simplest method and the space required is less than with belt drive. With direct drive the engine and dynamo must run at the same speed; this is a disadvantage because the desirable speeds of the two machines may not agree.
Since the usual engine speeds are slower than dynamo speeds, direct drive involves the use of a larger dynamo for a given output than would be necessary with belt connection, and involves a corresponding increase in cost and greater friction loss due to the rotation of larger and heavier parts.
Fig. 444.—Belt clamp for stretching belt and holding the ends while making joint. It consists of a stretching frame, the two ends of which are coupled by screwed bars; used for pulling the ends of a belt together with the proper tension, when lacing or joining the ends.