1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



431 



" On an uniform System of Screw Threads." Bv Josepli Wiitwjrtb, 

 Assoc. lust. C. E. 



Tlie suliject considered in this paper, is tlie importance of having a con- 

 stant thread for a given diameter in all screws used in fitting up steam en- 

 gines and other machinery. It is argued, that uniformity of thread woidd 

 be productive of economy, hoth in the use of screwing apparatus, and in the 

 consumption of bolts and nuts. The rclitting shop of a railway or steam 

 packet company affords a striking instance of the advantage to he derived 

 from the application of this princij)le. If the same system of screw threads 

 ivere common to the dittercnt engines, a single set of screwing tackle would 

 suffice for any repairs. No attempt appears to have been hitherto made to 

 attain this important object. Engineers have adopted their threads without 

 reference to a common standard. Any such standard must be in a great 

 measure arbitrary, and hence its absence may be accounted for. 



The author enters at some length into the consideration of the circum- 

 stances affecting the choice of a thread, with a view to show that it cannot 

 be determined by precise rules. The essential characters of the screw 

 thread are — pitch, depth, and form. The required conditions are — power, 

 strength, and durability. But the exact degree or proportion in which these 

 conditions are required, cannot be ascertained, and consequently the charac- 

 ters on which they depend cannot be fixed by calculation. An approxi- 

 mation may be made, but within a certain limit the decision is arbitrary. 

 The mutual dependence of the several conditions, and the relation subsisting 

 between the constituent characters, are noticed as having a tendency to 

 perplex in the choice of a thread. From the vagiieness of the principles 

 involved in the subject, a corresponding latitude was naturally to be expected 

 in the practical application of them, and accordingly, instead of that uni- 

 formity which is so desirable, there prevails a diversity so great as almost to 

 discourage any hope of its removal. The only mode in which this could be 

 effected, would he by a compromise; all parties consenting to adopt a me- 

 dium for the sake of common advantage. Tlie average pitch and depth of 

 the various threads used by the leading engineers, would thus become the 

 common standard, which would not only have the advantage of conciliating 

 general concurrence, but would in all probability approach very nearly to the 

 true standard for practical puiposes. 



The author then proceeds to describe the mode adopted by Messrs. Whit- 

 worth and Co., some years since, in selecting their threads upon this princi- 

 ple. An extensive collection was made of screw-bolts from the principal 

 workshops throughout England, and the average thread was carefully ob- 

 served for ditFerent diameters. The ^ inch, 4 inch, and \\ inch, were par- 

 ticularly selected, and taken as the fixed points of a scale by which the in- 

 termediate sizes were regulated, avoiding small fractional parts in the num- 

 ber of threads to the inch. The scale was afterwards extended to 6 inches. 

 The pitches thus obtained for angular threads were the following : — 



Above the diameter of 1 inch the same pitch is used for two sizes, to avoid 

 small fractional parts. The proportion between the pitch and the diameter 

 varies throughout the entire scale. Thus the pitch of the i inch screw is 

 ■^ of the diameter ; that of the i inch ^, of the 1 inch J, of the 4 inches 

 .jij, and of the 6 inches ^. The depth of the thread in the various speci- 

 mens is then alluded to. In this respect the variation was greater than in 

 the pitch. The angle made by the sides of the thread being taken as an 

 expression for the depth, the mean of the angle in 1 inch screws was found 

 to be about 55", which was also nearly the mean in screws of difterent dia- 

 meter. Hence it was adopted throughout the scale, and a constant propor- 

 tion was thus established between the depth and the pitch of the thread. 

 In calculating the former a deduction must be made for the quantity rounded 

 off, amounting to ^ of the whole depth, i. e. ^ from the top, and ^ from the 

 bottom of the thread. Making this deduction, the angle of 55° gives for the 

 actual depth rather more than J, and less than -j of the pitch. 



It is observed that the system of threads thus selected has already been 

 widely extended, demonstrating the practicability and advantage of the pro- 

 posed plan. The author then notices the obstacles to general uniformity 

 arising from the inconvenience which any change would occasion, in existing 

 establishments, and also from the imperfect screwing tackle in general use. 

 He anticipates as an important result of a combined effort to introduce uni- 

 formity, that screwing tackle generally would be much improved, and the 

 efficiency and economy of bolts and nuts be thereby increased. 



Me recommends also standard gauges of the diameters and threads, as 

 they would form a convenient adjunct to the screwing apparatus, and would 

 he applicable to other usefiJ purposes. 



Mr. Field claimed for the late Mr. Mandslay the credit of the first attempt 

 to introduce uniformity of thread — it was well known how incessantly his 

 attention and skill had been devoted to this object, and with what success 

 his efforts had been attended. He would at the same time accord great 

 merit to Mr. W'hitworlh, not only for his present effort to introduce a very 

 desirable measure, hut also for the general excellency of the screwing tackle 

 made under his directions. 



Mr. Seaward corroborated all that Mr. Field had advanced ; he had always 

 considered that to Mr. Maudslay the mechanical world was indebted for the 

 accuracy with which screws were now made. He considered the plan pro- 

 posed by Mr. Whitworth to be good, hut difficult of attainment in old-es- 

 tablished manufactories, where very extensive assortments of screwing tackle 

 already existed. 



The President concurred in the opinion, that it was to Mr. Maudslay's 

 well-known talent and skill that the mechanical world was indeljted for the 

 great improvements in the form of the screw, and the mode of its manufac- 

 ture ; but it was to Mr. Whitworth that the Institution was indebted for 

 having brought the subject before the meeting ; he trusted that this example 

 would not he lost upon the numerous members, who could contribute so 

 largely to the interests of the meetings, by recording the facts which came 

 under their notice in the course of their diurnal employments. 



" Jccount of the original construction and present state of the Plymouth 

 Breakwater." By William Stuart, M. Inst. C. E. 



In the year 1806, the Lords Commissioners of the Admiralty instructed 

 the late Mr. John Rennie, C. E., and .Mr. Whidhy, then Master-Attendant 

 at Woolwich Dock, to make a survey of Plymouth Soimd, with a view to 

 the construction of a breakwater for sheltering vessels. 



Their report was favourably received, but it was not until 1812 that the 

 works were commenced. 



Their plan consisted of a pier or breakwater 1700 yards in length, the 

 centre part of which, 1000 yards long, was straight, with an extension at 

 each end 250 yards in length, placed at an angle of 20" with the main body. 

 The top to be 30 feet wide at the level of 10 feet above the low water of an 

 ordinary spring tide : — the slope towards the sea to be 3 feet horizontal to 

 1 foot perpendicular, and on the land side 1 foot 6 inches horizontal to 1 

 foot perpendicular. 



By tlie middle of March, 1813, the work had been brought up in parts to 

 within 5 feet of low-water mark of spring tides — at this period 43,789 tons 

 of stone had been deposited — and in the month of March of the following 

 year sufficient shelter was afforded for ships of war to anchor in the Sound 

 instead of in Cawsand Bay. 



It was then determined to raise the whole structure to the height of 20 

 feet above low-water spring tides ; great exertions were made to complete it 

 rapidly, and during the week ending on the 24th of May, 1816, the quantity 

 of stone de]>osited amounted to 15,329 tons, which was the largest quantity 

 ever conveyed within the same space of time. 



Slight injury had frequently been received by the works whilst in progress, 

 but the storm of the night of the 19th of January, 1817, was the first which 

 materially affected them. The most destructive effects were, however, pro- 

 duced by' the storm of the 22nd and 23rd of November, 1824. On that 

 occasion the spring-tide rose 7 feet higher than usual ; and so great was its 

 power, that a length of 796 yards of finished work was completely over- 

 turned, and the remaining parts slightly injured. 



It was observed, that the effects of this storm left the slope from low- 

 water mark upward at about 5 feet horizontal to 1 foot vertical. It was 

 therefore determined to adopt that angle of inclination for the exterior or 

 sea side, and a slope of 2 to 1 for the inside. The centre line of the break- 

 water was also removed 39 feet 6 inches towards the north, and the top 

 width was increased to 45 feet. 



The works continued upon this scale until 1830, when a fore-shore was 

 added of a width of 50 feet at the toe of the south slope at the west end, 

 and of 30 feet at the east end of the main arm : for this purpose 600,000 

 tons of stone were deposited. 



The extreme western erd of the breakwater was then, after more alte- 

 rations and extensions, terminated by a circular head, with an inverted arch 

 as a foundation for a light-house, now constructing under the direction of 

 Messrs. Walker and Burges. 



In consequence of further injuries from storms in 1838, when large quan- 

 tities of stones of 16 or 20 tons' weight each were torn from below low 

 water, and carried completely over the top of the Breakwater, a further ex- 

 tension of the fore-shore was made, and a projecting buttress built to secure 

 the foot of the south slope, to afford additional security to the light-house, 

 and to prevent the stones from the fore-shore being carried over to the 

 north side. 



From the time of its commencement in August, 1812, until the 31st of 

 March, 1841, there had been deposited upon this work 3,369,261 tons of 

 stone. 



Cost. — The expenditure upon the whole work, when completed, will, it is 

 estimated, be within one million and a half sterling. The main body of the 

 work is composed of blocks of limestone from the quarries of Oreston, ad- 

 joining the harbour of Catwater. They were deposited from vessels con- 



