THE TURN OF THE SCREW: OPTIMAL DESIGN OF AN ARCHIMEDES SCREW By Chris Rorres1 ABSTRACT: The geometry of an Archimedes screw is governed by certain external parameters (its outer radius, length, and slope) and certain internal parameters (its inner radius, number of blades, and the pitch of the blades). The external parameters are usually determined by the location of the screw and how much water is to be lifted. The internal parameters, however, are free to be chosen to optimize the performance of the screw. In this paper the inner radius and pitch that maximize the volume of water lifted in one turn of the screw are found. The optimal parameter values found are compared with the values used in a screw described by the Roman architect and engineer Vitruvius in the ®rst century B.C., and with values used in the design of modern Archimedes screws. INTRODUCTION (or ``pitch'') of these blades was equal to the circumference of the cylindrical core. Finally, an outer cylindrical covering One of the oldest machines still in use is the Archimedes (the ``outer cylinder'') of wooden planks was nailed to the screw, a device for lifting water for irrigation and drainage helical blades. Liquid pitch was smeared over all of the parts purposes. Its invention has traditionally been credited to Ar- during their assembly to make the screw watertight. The rigid chimedes (circa 287±212 B.C.). For example, Diodorus Si- screw was mounted so that it could be rotated along its length culus (Greek historian, circa ®rst century B.C.) writes (Fig. 2) and was tilted in the direction of the hypotenuse of a 3-4-5 triangle (a ``Pythagorean right-angled triangle,'' as Vi- men easily irrigate the whole of it [an island in the delta of truvius calls it). Its bottom end was immersed in a body of the Nile] by means of a certain instrument conceived by water (the ``lower reservoir''), and through the rotation of the Archimedes of Syracuse, and which gets its name [cochlias] screw water was lifted to the top end of the screw into an because it has the form of a spiral or screw. ``upper reservoir.'' When an Archimedes screw is tilted, ``buckets'' that can And from Athenaeus of Naucratis (Greek historian, circa A.D. trap water are formed between the blades. These buckets ap- 200): pear to move upward when the screw is rotated, carrying the water within them. The screw collects water from the lower The bilge-water [of the ship Syracusia], even when it be- reservoir, where the buckets are formed, and empties it into came very deep, could easily be pumped out by one man the upper reservoir, where the buckets are unformed. When with the aid of the screw, an invention of Archimedes. operated manually it is rotated by a crank (Fig. 3) or by a man walking around the circumference of the outer cylinder in a (See references under Diodorus Siculus and Athenaeus of Nau- treadmill manner. cratis.) Archimedes, however, made no reference to it in his In modern industrial screws, the outer cylinder is usually extant works [cf. Heath (1897) and Dijksterhuis (1938)], and ®xed and the blades attached to the inner cylinder are rotated it may be that he simply transmitted its knowledge from Egypt within it (Fig. 4). This allows the top half of the outer cylinder (where it is believed he studied in Alexandria under the stu- to be eliminated so that a stationary trough is formed from the dents of Euclid) to Syracuse (his native city in Sicily). On the bottom half of the outer cylinder. Such a construction permits other hand, in defense of Archimedes: no mention of the de- easy access to the interior of the screw, in order to remove vice exists before his time, its design involves the type of debris and for routine maintenance. In addition, the stationary geometry in which he excelled, and his abilities as an inventor outer cylinder relieves the moving blades and inner cylinder of mechanical and military machines are well documented. of some of the weight of the water. A disadvantage of this The Roman engineer and architect Vitruvius gave a detailed design is that water can leak down through the small gap be- and informative description of the construction of an Archi- tween the moving blades and the stationary trough. However, medes screw in his De Architectura, written in the ®rst century this leakage can be considered an advantage in that it allows B.C. (See reference under Vitruvius.) Vitruvius's description the screw to drain when it stops rotating. contributed greatly to keeping the device well known through- out the ages, and the particular screw he described will be The Archimedes screw has had a resurgence in recent years used throughout this paper as a test case. because of its proven trouble-free design and its ability to lift Vitruvius's screw began with a tree trunk shaped into a cy- wastewater and debris-laden water effectively. It has also lindrical core (the ``inner cylinder'') whose length is 16 times its diameter (Fig. 1). On this cylindrical core eight intertwined helical blades (also called ``¯ights'' or ``starts'') were con- structed by nailing withes (slender ¯exible willow twigs) to- gether up to a height equal to the radius of the core. The period 1Prof., Dept. of Math. and Comp. Sci., Drexel Univ., Philadelphia, PA 19104. E-mail: [email protected] Note. Discussion open until June 1, 2000. To extend the closing date one month, a written request must be ®led with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on February 19, 1998. This paper is part of the Jour- nal of Hydraulic Engineering, Vol. 126, No. 1, January, 2000. qASCE, FIG. 1. Vitruvius's Eight-Bladed Screw as Described in his De ISSN 0733-9429/00/0001-0072±0080/$8.00 1 $.50 per page. Paper No. Architectura. Diagram by Morris Hicky Morgan (Vitruvius Ref- 17708. erence) 72 / JOURNAL OF HYDRAULIC ENGINEERING / JANUARY 2000 FIG. 2. Pro®le of Three-Bladed Archimedes Screw proved valuable in installations where damage to aquatic life must be minimized. The purpose of this paper is to examine the optimal design of an Archimedes screw. By this is meant that geometry that will maximize the amount of water delivered to the upper res- ervoir in one turn of the screw. This optimization will be per- formed under the assumption that the outer radius of the screw is speci®ed, for clearly the amount of water lifted in each revolution can be continually increased by continually increas- ing this radius. The amount of water lifted per unit time can also be in- creased by increasing the rotational velocity of the screw. However, there is a practical limit to how fast one can rotate the screw. A handbook on the design and operation of Archi- medes screws (Nagel 1968, p. 37) states that, based on ®eld experience, the rotational velocity of a screw in revolutions per minute should be no larger than 50/D2/3, where D is the diameter of the outer cylinder in meters. Thus a screw with FIG. 3. Two-Bladed Archimedes Screw in Use in Modern an outside diameter of 1 m should have a maximum rotational Times in Egypt to Irrigate Fields velocity of 50 rpm. If the screw is rotated much faster, tur- FIG. 4. Seven 96-Inch-Diameter, 3-Flight Screw Pumps That Lift Wastewater from Pretreatment Section to Balance of TreatmentPlant in Memphis, Tenn. [Each Pump Has Capacity of 19,900 gal./min (Courtesy Lakeside Equipment Corp., Bartlett, Ill.)] JOURNAL OF HYDRAULIC ENGINEERING / JANUARY 2000 / 73 bulence and sloshing prevent the buckets from being ®lled and volume of one bucket. The volume of one bucket is denoted the screw simply churns the water in the lower reservoir rather by VB. It is a complicated function of N, K, L, Ro and Ri. than lifting it. The restriction L # 2pRo /K given above on the pitch L of the screw needs some discussion. In order for water to be PROBLEM FORMULATION trapped in the screw, it is necessary that the sinusoidal curve in Fig. 5 de®ning the outer edge of a blade tilts downward as Fig. 5 is a pro®le view of a segment of an Archimedes screw it crosses the axis of the screw. In terms of the angles u and showing how the buckets are formed between pairs of adjacent a in the ®gure, it is necessary that u # a or, equivalently, blades. The buckets move up the screw as it is rotated clock- tan u # tan a. Now tan u = K by de®nition, and tan a = wise when viewed from the upper reservoir. (Throughout this Ro(2p/L) since the sinusoidal curve has amplitude R0 and paper the screws are oriented for clockwise rotation). The in- period L. The condition tan u # tan a is thus K # Ro(2p/L), ner and outer edges of each blade determine two sinusoidal which is just L # 2pRo /K. curves of the same period (or pitch) and phase. One has an Fig. 5 also shows the angle b that the sinusoidal curve de- amplitude equal to the inner radius of the screw and the other ®ning the inner edge of a blade makes with axis of the screw. u has an amplitude equal to the outer radius. The angle that Because the inner sinusoidal has amplitude Ri and period L, u the screw makes with the horizontal determines its slope tan .