General Description.—As has been said, the Dipper Dredge “ digs ” like the familiar steam shovel. The bucket, which is essentially a scoop with heavy teeth and a flap bottom, is attached to the end of the dipper stick, which is carried by the boom at or near the centre of the latter. The boom is supported by “A” frame and back guys as in the grapple, except that, frequently, the “A” frame is tilted forward, when it is termed the “shear legs.” The boom is necessarily of heavier construction than that of the grapple and, with the “A” frame, is set well forward at the very bow of the hull.
The bucket obtains its load by digging into the bottom under the impetus of dipper stick and bucket wire, which runs over a sheave in the boom head and thence to the main engine. The boom is spying in one of two ways; either as in the grapple by means of two bucket wires, or by bull wheel and swinging engine, the latter being the more common. The hull is stiffer and the spuds heavier than those of the grapple, because of the greater strains. The dredge is held in position by three spuds, and advances by grounding her dipper and stressing the backing chain. As a rule, the crew is quartered on the dredge. The scows are handled as on grapples.
The Bucket.—The bucket is an open steel cylinder, provided with a hinged flap bottom, a handle or bail and a reinforced cutting edge or lip, to which teeth are attached. Fig. 9 is a picture of a Bucyrus Dipper Dredge, swinging a 6 cubic yard bucket.
The bucket hoisting wire is made fast to the centre of the bail, and the backing chain, which draws bucket and dipper stick back toward the hull, is fastened either to the
rear face of the bucket or to the dipper stick near the bucket. The bottom door or floor of the bucket is held closed by a latch, which locks automatically by virtue of its bevelled end when the door is forced upward and closed by the water pressure. The latch is drawn back to open the door by means of a line extending from it to a lever at a point near the fulcrum, and a second line from the end of the lever to the cranesman stationed at the heel of the boom. In the larger buckets the latch rests on roller bearings to facilitate its movement and, in the recent, large capacity, high powered machines, is steam operated. The teeth are usually from three to five in number, with tool steel points, and are detachable for sharpening and renewing.
Boom, Dipper Stick, “A” Frame and Back Guys.—The boom, as in the grapple, is hung from the “A” frame by a fixed topping fall, but is usually at a flatter angle with the horizontal than that of the grapple. The “A” frame, when vertical, is approximately in the plane of the heel of the boom, but, when inclined, the point of bearing on the deck may be some distance aft of the boom heel. For the same topping-fall stress, the stresses in “A” frame and back guys are greater in the case of the inclined shear legs than in that of the vertical “A” frame. The boom, necessarily, is set well forward in order that the dipper stick may clear the bow in all positions. The back stays, more particularly in those dredges having inclined “A” frames or shear legs, are frequently tension members only, without back legs or struts. There is a structural economy in the vertical “A” frame, in that it and the gallows frame may be designed as a unit frame, whereas the inclined “A” frame necessitates a distinct and independent gallows frame.
The boom is the most difficult part of the dredge to design. Some of the very heavy loads to which it is subjected are indeterminate, principally those caused by starting to swing the boom before the bucket is clear of the water, or even while still in the mud, and by sudden stoppage and reversal of swing. The principal boom stresses are again combined compression and bending, but, in this instance, the bending is due to live as well as dead load and is very much greater. In addition there is, in those booms swung by a bull wheel at the heel, a horizontal bending movement, caused by the rotation of the bull wheel
and maximum at the boom heel casting. Furthermore, the side thrust of the dipper stick requires considerable lateral boom stiffness. To investigate understandingly the vertical bending stress in the boom, a knowledge of the action and control of the dipper stick is necessary.
The stick has two movements, one of translation with respect to and through the boom at or near its mid point, and the other of rotation in a vertical plane through an arc centred at the same point. Referring to Fig. 10, the dipper may be pulled either toward the end of the boom by the main engine wire A, or back toward the hull by the backing chain B. The motion of the stick through the boom is controlled by two band friction wheels C on the boom, keyed to a shaft carrying a pinion which meshes into a rack on the under side of the dipper stick, so that the stick may be held fast at the boom at any point of its length, at the same time being free to rotate about that point. The stick is pulled up through the boom by the tension in hoisting wire or backing chain, or both, and drops down through the boom by gravity alone. Many dippers, however, are equipped with a so-called “crowding engine,” which is mounted on the boom and drives the pinion meshing with the rack of the stick, so that the stick can be pushed or pulled through the boom. By this means, the dipper can be thrust out beyond the end of the boom to an increased reach.
It is apparent from the stress diagrams (fig. 10-b) that the digging power, or the thrust at the bucket perpendicular to the dipper stick, varies inversely as the length L of stick below the boom and the angle θ between boom and stick; and that the compression in the dipper stick is maximum when L and θ are maximum. This, then, is the critical loading of the stick and, knowing the greatest pull of which the main engine is capable, the maximum compressive stress in the stick follows. Its length will usually be such as to necessitate the use of long column formula in its design.
It is entirely possible, too, that the dipper stick be subjected to tension, which, although insufficient to influence the choice of section, is enough to require a test of the stick details for resistance to the tensile stress involved, which will be the weight of the loaded bucket in air. This condition obtains when, through faulty operation, or the parting
of the main hoisting wire, or slipping main engine frictions, the full load of the bucket and contents is suspended from the boom by the dipper stick, held by its frictions at the boom. If there be a crowding engine, the same tension may easily exist through the agency of that engine. In addition, there is a certain amount of twisting to be resisted.
The direct stress in boom and topping fall may be found graphically as in fig. 12. The forces bc and ac, fig. 12-B, representing the bucket wire tension, are obviously equal in amount, as are ac and de of fig. 12-C, because the wire passes over a sheave in the boom head, and are measured by the pull of the main engine. The boom compression is maximum when ac is perpendicular to cd, and the topping fall tension cd increases with the angle θ. It must be remembered in the design of the latter, that the angle θ is not limited by the vertical position of the bucket wire, as the bucket may easily be thrust out beyond the end of the boom, more especially by the use of a crowding engine. The dead load stress in the topping fall is obtained as described under grapples.
There are four values of bending moment to be considered in the boom: a, the positive vertical B.M. caused by the suspension of the loaded bucket and dipper stick as mentioned above; b, the negative vertical B.M., caused by the thrust of the dipper stick when perpendicular to the boom; c, the positive dead B.M. due to the weight of the boom itself; and d the horizontal B.M. as a result of the bull-wheel rotation. The critical condition of stress in the boom, then, is the greatest possible combination of direct compression and compressive fibre stress due to vertical and horizontal bending moment, not omitting to investigate and provide for the fibre tension. These stresses, in conjunction with those due to the indeterminate loads previously referred to and with the physical considerations influencing the shape and location of some of the members, result in a sort of compromise design, a and b are maximum at the point of intersection of dipper stock and boom; c at the centre of the boom; and d at the heel casting. The horizontal B.M. d is equal to the pull of the swinging engine multiplied by the radius of the bull wheel. It has little bearing upon the choice of the boom
section, but is an important destructive agent acting upon the heel casting and the boom structure immediately adjacent thereto, and one warranting complete investigation and the provision of ample resistance.
Some dippers are rigged with a single sheave purchase in the bucket wire, fig. 12-A; i.e. the end of the wire is attached to the boom end, from whence it passes down to a single-sheave block fastened to the bail of the bucket, thence up over the boom-head sheave and to the main engine. It is apparent that the result of such a rigging is to double the lifting power of the engine and to halve the lifting speed and that, in dippers of the same digging power, the boom stress in the machine which is purchase rigged will be less than that in the direct wire dredge (Fig. 10).
The boom is a relatively large member, having considerable depth at the dipper stick and tapering toward both ends, and is of plate, girder or latticed truss construction.
The stress in “A” frame or shear legs and back legs or stays are found as for grapples in Chapter I.
Spuds.—The two bow spuds are set either in “outside” or “through” wells. They are of heavy section and are subject to considerable bending moment occasioned by the reaction of the forward thrust of the dipper. Some dippers are rigged to “pin up,” a term applied to the process of maintaining the dredge upon an even keel while digging, by transferring part of the weight of the machine to the two bow spuds, so that it is not dependent upon the stability of the hull to resist transverse oscillation. In pin-up dredges, the fore spuds are provided with sheaves both top and bottom. A wire attached to the spud-well housing, running down around the toe sheave and thence up to the drum of the spud hoist, raises the spud, and a second wire of larger diameter, fastened at the same place and leading up over the top sheave and down to the drum, becomes taut when the dredge attempts to list to that side, tending to force the spud more deeply into the bottom.
The dredge advances by grounding her bucket well ahead of the bow, raising the two bow spuds clear of the bottom and stressing the backing chain, which pulls the machine toward the dipper. The stern spud remains in the mud during the operation, and, having a slotted well, slowly inclines forward as the dredge progresses. It is
called the “walking-spud” or “trailing-spud” on this account. The spuds are raised by spud hoist and gallows frame, or by a rack on the spud engaging a pinion at the deck level, obviating the necessity of a gallows frame.
The Machinery.—In dipper dredges having two bucket wires to swing the boom and raise the bucket, the main engine is a two cylinder horizontal type, with two drums
as in the grapple. If the boom is controlled by swinging engine and bull wheel, the main engine may have but one hoisting drum. In some machines, the drum cylinder is of large diameter for part of the width, stepping down to a smaller diameter for the balance, the object being to increase the pull on the bucket wire during the early stage of digging when the bucket is loading, after which, the wire coils upon the larger drum surface, accelerating the raising of the dipper. Such a device is termed a differential drum, and is used to good advantage with the purchase rigged dipper. In addition to the main and swinging engines, complete dipper control requires an engine for handling the backing chain which draws the bucket back toward the hull. Some machines are equipped also with a crowding engine mounted upon the boom, with the function of dipper stick motion as previously described. The machinery for spud and scow handling is similar to that of the grapple.
The dredge President, American Dredging Company, pictured on page 38, is a “pin-up” machine, with direct wire rigging, bull wheel and crowding engine, hull 105 ft. x 39 ft. 8 in. x 10 ft. 7 in., 2 cyl. hor. main engine 14 x 16, and a 4½ yd. dipper.
The Hull.—In general dimensions and construction details, the hull is very similar to that of the grapple. There is, however, a need for greater stiffness due to the thrust of the dipper and the extreme forward mounting of the boom. In some types, e.g., the Bucyrus built machines, the longitudinal trusses are steel and of great depth, reaching from the cross keelsons to the roof of the deck house; and tied horizontally by a top lateral truss. There is, too, relatively less stability forward than in grapples to resist listing when the bucket is swung out over the scow, from which it is apparent that the same hull will carry a grab bucket of larger capacity than a dipper.
The bow structure of the hull must be well stiffened and the truss and superstructure design there is dependent in some measure upon the locating of the bull wheel, which may be upon the deck or elevated to the plane of the roof
of the deck house. In the latter position, it exerts a more direct pull upon the boom, thereby reducing the lateral stresses in that member.
Operation.—Dippers require two men to operate the boom, dipper stick and bucket, in addition to the usual crew of engineer, firemen, oilers and deck hands. One, the “operator” or “runner,” controls the bucket hoist, backing chain and boom motion through the throttles and frictions of the main, backing and swinging engines. The other, the “cranesman” or “dipper tender,” is stationed at the boom heel and regulates the dipper stick frictions on the boom and opens the bucket.
In the process of digging, the runner slacks the bucket wire or wires, permitting it to drop into the water, at the same time pulling it back toward the hull by stressing the backing chain. The cranesman releases the dipper stick frictions so that it falls through the boom until the dipper rests on the bottom. The runner releases the backing chain and stresses the hoisting wire, and the dipper tender applies his friction bands, gripping the dipper stick at the boom. The bucket, therefore, cuts its way forward in an arc of radius equal to that portion of the dipper stick below the boom until it is loaded and clear of the mud, whereupon
the cranesman releases the stick, which shoots up through the boom. The runner swings the boom until the bucket is suspended over a pocket of the scow and the cranesman pulls the latch string, dumping the bucket.
Application of the Type.—To the need of the American contractor for a simple, inexpensive and versatile machine, the dipper dredge owes its rapid development in this country. It is very efficient in hard material, provided the depth is not excessive, and is used to advantage in canal work through solid ground containing stumps and roots, in the dredging of previously blasted or loose rock
and in the removal of old filled cribs, old foundations, sunken wrecks and stone dikes. Thus it is a most capable machine, with a wide application. The conditions of uniformly hard materials and moderate requisite depth of channel on the Great Lakes have occasioned an extensive use of the dipper there.
High-powered Dipper Dredges.—The earliest dipper dredge of large size was the 12 yard machine ONONDAGA, owned and operated in New York Harbor about 1904 by the contractors Hughes Brothers & Bangs. Since then, and prior to 1904, the largest exponents of the type were probably the two 10 yard machines used on the Cape Cod Canal, built by the Atlantic Equipment Co., and the 15 yard dredge TOLEDO, built by the Bucyrus Company.
At the time of the closing of the Panama Canal by the first large slide, the largest dippers in use there were 5 yard. It was decided that the only type of machine suitable for the removal of this huge mass of broken rock and earth, containing pieces of rock of all sizes and lacking all uniformity of formation, was a large capacity and high-powered dipper dredge. The decision resulted in the construction by the Bucyrus Company of three 15 yard dipper machines, the GAMBOA, PARAISO and CASCADAS, the latter being the last built and arriving at the Isthmus in October of 1915. Although open to criticism perhaps as to the abnormally high cost of maintenance, attributed by some engineers to defects in the main hoisting wires, the dipper arms and the spuds, both the builders and the management were justified by the splendid performance of the machines and by the fact that the first two were in large part copies of the TOLEDO because of the lack of time to prepare new designs. Each of the three dredges had a demonstrated capacity of more than 3,000,000 cubic yards per year. They are all pin-up machines, with steel hulls. The principal statistics of the above dredges are given in the Table, page 45.
N. B. The CASCADAS differed principally in the width of hull, which was 55 ft., in the depth, 15′-6″, and in the use of a gallows frame to raise the spuds without the use of sheaves in the spud toes, which proved objectionable because of the cable abrasion due to the rock. Several improvements were also made in the machinery and in other details as dictated by the experience with the other two.