Machine design - Benjamin

The present book represents the consolidation of two texts on this subject, Benjamin's Machine Design and Hoffman's Elementary Machine Design.

As now arranged, the book serves two purposes: That of a text for the classroom, embodying the theory and practice of design, and that of a reference book for the drafting room, illustrating the design of complete machines.

The authors recognize the fact that there are two methods of teaching this subject, one by details separately treated as elements, one by a consideration of the complete machine, i.e., one method is synthetic and one analytic. It is believed that this book will afford a means of using either method or both combined.

Some important additions to the text are worthy of mention. Chapter II, on Materials, has been rewritten. Much additional matter on the subject of cast-iron frames has been introduced, involving the results of numerous experiments. The theoretical and experimental strength of steel tubes under collapsing pressures is quite fully discussed and additional data are given on the failure of pipe fittings.

Other subjects which receive in this volume fuller treatment than heretofore are Flat plates, Crane hooks. Leaf springs. Bearings, both plain and rolling. Clutches, Gear teeth and Belting.

General Principles of Design. - The working or moving parts should be designed first and the frame adapted to them.

The moving parts can be first arranged to give the motions and velocities desired, special attention being paid to compactness and to the convenience of the operator.

Novel and complicated mechanisms should be avoided and the more simple and well-tried devices used.

Any device which is new should be first tried in a working model before being introduced in the design.

The dimensions of the working parts for strength and stiffness must next be determined and the design for the frame completed. This may involve some modification of the moving parts.

In designing any part of the machine, the metal must be put in the line of stress and bending avoided as far as possible.

Straight lines should be used for the outlines of pieces exposed to tension or compression, circular cross-sections for all parts in torsion, and profile of uniform fiber stress for. pieces subjected to bending action.

Superfluous metal must be avoided and this excludes all ornamentation as such. There should be a good practical reason for every pound of metal in the machine.

An excess of metal is sometimes needed to give inertia and solidity and prevent vibration, as in the frames of machines having reciprocating parts, like engines, planers, slotting machines, etc.

Mr. Oberlin Smith has characterized this as the "anvil" style of design in contradistinction to the "fiddle" style.

In one the designer relies on the mass of the metal, in the other on the distribution of the metal, to resist the applied forces.

A comparison of the massive Tangye bed of some large high- speed engines with the comparatively slight girder frame used in most Corliss engines, will emphasize this difference.

It may be sometimes necessary to waste metal in order to save labor in finishing, and in general the aim should be to economize labor rather than stock.

The designers should be familiar with all the shop processes as well as the principles of strength and stability. The usual tendency in design, especially of cast-iron work, is toward unnecessary weight.

All corners should be rounded for the comfort and convenience of the operator, no cracks or sharp internal angles left where dirt and grease may accumulate, and in general special attention should be paid to so designing the machine that it may be safely and conveniently operated, that it may be easily kept clean, and that oil holes are readily accessible. The appearance of a machine in use is a key to its working condition.

Polished metal should be avoided on account of its tendency to rust, and neither varnish nor bright colors tolerated. The paint should be of some neutral tint and have a dead finish so as not to show scratches or dirt.

Beauty is an element of machine design, but it can only be attained by legitimate means which are appropriate to the material and the surroundings.

Beauty is a natural result of correct mechanical construction but should never be made the object of design.

Harmony of design may be secured by adopting one type of cross-section and adhering to it throughout, never combining cored or box sections with ribbed sections. In cast pieces- the thickness of metal should be uniform to avoid cooling strains, and for the same reason sharp corners should be absent. The lines of crystallization in castings are normal to the cooled surface and where two flat pieces come together at right angles, the interference of the two sets of crystals forms a plane of weakness - at the corner. This is best obviated by joining the two planes with a bend or sweep.

Rounding the external corner and filleting the internal one is usually sufficient. Where two parts come together in such a way as to cause an increase of thickness of the metal there are apt to be "blow holes" or "hot spots" at the junction due to the uneven cooling.

Machine Supports. - The fewer the number of supports the better. Heavy frames, as of large engines, lathes, planers, etc., are best made so as to rest directly on a masonry foundation. Short frames as those of shapers, screw machines and milling machines, should have one support of the cabinet form. The use of a cabinet at one end and legs at the other is offensive to the eye, being inharmonious. If two cabinets are used provision should be made for a cradle or pivot at one end to prevent twisting of the frame by an uneven foundation. The use of intermediate supports is always to be condemned, as it tends to make the frame conform to the inequalities of the floor or foundation on what has been aptly termed the caterpillar principle.

A distinction must be made between cabinets or supports which are broad at the base and intended to be fastened to the foundation, and legs similar to those of a table or chair. The latter are intended to simply rest on the floor, should be firmly fastened to the machine and should be larger at the upper end where the greatest bending moment will come.

The use of legs instead of cabinets is an assumption that the frame is stiff enough to withstand all stresses that come upon it, unaided by the foundation, and if that is the case intermediate supports are unnecessary.

Whether legs or cabinets are best adapted to a certain machine the designer must determine for himself.

Where two supports or pairs of legs are necessary under a frame, it is best to have them set a certain distance from the ends, and make the overhanging part of the frame of a parabolic form, as this divides up the bending moment and allows less deflection at the center. Trussing a long cast-iron frame with iron or steel rods is objectionable on account of the difference in expansion of the two metals and the liability of the tension nuts being tampered with by workmen.

Machine Frames. - Cast iron is the material most used but steel castings are now becoming common in situations where the stresses are unusually great, as in the frames of presses, shears and rolls for shaping steel.

Cored vs. Rib Sections. - Formerly the flanged or rib section was used almost exclusively, as but a few castings were made from each pattern and the cost of the latter was a considerable item. Of late years the use of hollow sections has become more common; the patterns are more durable and more easily molded than those having many projections and the frames when finished are more pleasing in appearance.

The first cost of a pattern for hollow work, including the cost of the core-box, is sometimes considerably more but the pattern is less likely to change its shape and in these days of many castings from one pattern, this latter point is of more importance. Finally, it may be said that hollow sections are usually stronger for the same weight of. metal than any that can be shaped from webs and flanges.

Resistance to Bending. - Most machine frames are exposed to bending in one or two directions. If the section is to be ribbed it should be of the form shown in Fig. 3. The metal being of nearly uniform thickness and the flange which is in tension having an area three or four times that of the compression flange. In a steel casting these may be more nearly equal. The hollow section may be of the shape shown in Fig. 4, a hollow rectangle with the tension side re-enforced and slightly thicker than the other three sides. The re-enforcing flanges at A and B may often be utilized for the attaching of other members to the frame as in shapers or drill presses. The box section has one great advantage over the I section in that its moment of resistance to side bending or to twisting is usually much greater. The double I or the U section is common where it is necessary to have two parallel ways for sliding pieces as in lathes and planers. As is shown in Fig. 5 the two I's are usually connected at intervals by cross girts.

Besides making the Cross-section of the most economical form, it is often desirable to have such a longitudinal profile as shall give a uniform fiber stress from end to end. This necessitates a parabolic or elliptic outline of which the best instance is the housing or upright of a modern iron planer.

Resistance to Twisting. - The hollow circular section is the ideal form for all frames or machine members which are subjected to torsion. If subjected also to bending the section may be made elliptical or, as is more common, thickened on two sides by making the core oval. See Fig. 6. As has already been pointed out the box sections are in general better adapted to resist twisting than the ribbed or I sections.

Frames of Machine Tools. - The beds of lathes are subjected to bending on account of their own weight and that of the saddle and on account of the downward pressure on the tool when work is being turned. They are usually subjected to torsion on account of the uneven pressure of the supports. The box section is then the best; the double I commonly used is very weak against twisting. The same principle would apply in designing the beds of planers but the usual method of driving the table by means of a gear and rack prevents the use of the box section. The uprights of planers and the cross rail are subjected to severe bending moments and should have profiles of uniform strength. The uprights are also subject to side bending when the tool is taking a heavy side cut near the top. To provide for this the uprights may be of a box section or may be reinforced by outside ribs.


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