Why Chromium Bores Reduce Wear
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Unusually High Mileages, with Little or No Need for Replacement Parts, Achieved by Coming'.
Chromium-bore Engines
IS chromium plating of cylinder bores as successful in reducing engine wear
as its sponsors would imply? If so, how and why has this great technical advance been achieved? Authenticated figures supplied by operators answer the lust question. A study of the methods used by Commer Cars, Ltd., Luton, and of the reasons for them, provides the technical background required to answer the second question.
Although full-length chromiumfinished bores were not used until 1948, when they were incorporated in the new Commer 109 b.h.p. underfloor engine, extensive research into the process was conducted during the late 1930s by engineers of the Rootes Group. They worked in conjunction with the Van Der Horst Corporation, but it was not until prototypes had operated successfully for many years that the innovation was passed on to operators.
Special Plating Process
The final deposit of chromium on the bores is .0043 in. thick-200 times as thick as decorative plating. A special process is used in which the Metal is etched to create a complete network of minute fissures in the surface. This etched surface has particular importance in solving the problem of bore wear.
Cylinder blocks are bored slightly oversize, so that the chromium will bring them back to nominal. Surplus oil and grease are removed in a degreasing bath, and the blocks are mounted on a holding fixture which is also arranged to attach rigidly a lead anode passing down each bore.
The cast-iron surfaces of the bores are then prepared in an etching bath. This is important, because of the necessity of securing good adhesion between iron
1324 and chromium. The etched blocks are conveyed to the plating bath, which contains a mixture of chromic acid and chromic oxide. Plating continues for some hours, and the chromium deposited is thicker than that which will eventually be required.
If the bores were machined smooth immediately after plating, the surface would not hold lubricating oil. It is, therefore, necessary to produce a porous surface on the chromium, and this is obtained by further etching, after which the bores are honed to the correct size. Finally, any chromium which may have lodged in the pores during honing is removed in a steam booth, where steam is pumped through the bores.
In this way, the five well-established causes of cylinder-bore wear are tackled.
Any corrosive product that tends to attack the cylinder walls finds great difficulty in adhering, because of the very low coefficient of friction of chromium, and is wiped away by the scraper rings before doing damage. Chromium has many times greater resistance to these acids than has normal cast iron.
Abrasion is negligible, for chromium is rated next to diamond in hardness. When measured on the Mohs Scale of hardness, based on a diamond hardness of 10, the figure for chromium is 9, for tungsten is 7.5, cobalt 5.5, iron 4.5, platinum 4.3, and so on. Any smaLl particle trapped between the cylinder wall and the moving components i s .rubbing against a surface many times harder than cast iron and its abrasive effect is greatly reduced.
Scuffing is also reduced, because of the low wear characteristics of chromium when in contact with cast iron. On this basis, when expressed in relative figures, chromium is 0.43, casehardened steel is 1.21, tool steel is 1.56, and austenitic steel is 3.55.
With porous chromium, the minute pits and crevices produced during etching add up to nearly 40 per cent, of the total bore area, so the contact area between the moving parts is proportionally reduced.
The very low frictional values between chromium hores.and the metals in contact reduce the tendency for welding to take place. The thermal conductivity of chromium is 40 per cent. higher than that of cast iron or steel, and this permits rapid heat flow. The limit of surface temperature is raised, because electrolytic chromium has a melting point 800° F. above that of cast iron and 500° F. higher than that of steel. There is also less tendency for fusion to take place •between the chromium bores and the cast-iron rings or aluminium pistons.
Lubrication is greatly assisted by the pits and crevices over the length of the bore, which retain the oil under all operating conditions.
Disintegration of the surface is prevented, because chromium plating creates an .overlay which seals the castiron base metal, Flour Millers' Experience'
What this process means in terms of improved engine life can best be told by operators. Among themare Cranfield Bros., Ltd., Ipswich, the flour millers.
They bought their first Commersthree underfloor-engincd petrol vehicles —from Prentice Aircraft and Cars, Ltd., in the summer of 1949. These were two 5-tonners and a 7-tonner, which, at the time of my visit, had covered 196,000 miles, 210,000 miles and 213,000 miles respectively. Each was still adding 1,000 miles every week to these figures. Each had been decarbonized only once, at about 120,000 miles, when cylinderbore wear was .001 in. and crankshaft wear was from .0005 in. to .001 in.
Esso S.A.E. 5 engine oil is used all the year round and yields an average of 1,600 m.p.g. It is changed every 4,0005,000 miles. The 5-tonners have averaged 11-12 m.p.g. of fuel and the 7-tonner, 10 m.p.g.
Cranfield's now have 21 Commers, all maintained by Prentice Aircraft. They comprise two petrol-engincd 5-tonners, six petrol 7-tonners, four petrol articulated 12-tonners, six oil-engined 7-tonners, and three oil-engined articu lated 12-tonners. The oilers, which incorporate the Rootes TS3 unit, were among the earliest of their kind to be manufactured.
The seven 12-ton tractors are operated in conjunction with 10 Hands semi-trailers. This combination has proved to be more economic than the 5-ton or 7-ton units, and more are likely to be purchased. Some smaller vehicles will have to be retained, because the premises of many customers are rather cramped.
A Seven-make Fleet
Cranfield's total fleet of 48 vehicles contains 19 Bedfords of various types, three Austins, two Morris, a Dodge, an Albion and a Maudslay. Six Bedfords and the Dodge are based at Strood, Kent, but the rest are at Ipswich.
They carry flour from Ipswich within a 100-mile radius, which enables them to return to base each day, but there are also a few two-day workings. Deliveries to most customers are weekly.
The 5-tonners carry 100 140-16. bags of flour on each trip, the 7-tonners, 130 bags, and the 12-tonners, 200. The traffic is almost entirely one way, with empty return workings, because although about a quarter of the wheat used in the mill is grown in East Anglia, it is delivered almost entirely in the vehicles of other merchants, which then return with by-products from the mill. Forty-six of the 62 forward-control Commers supplied by Cathedral Garage, Ltd., to the Metal Agencies Co., Ltd., Bristol, have chromium cylinder bores, and the engines have between them already exceeded 6/m. miles. No major overhauls have been needed. Cylinder heads arc exchanged at 50,000 miles and gearboxes are replaced at 100,000 miles.
Typical bore wear in the engine of a 5-tonner was .001 in. at 137,000 miles. At that mileage the crankshaft was still within the maker's 'tolerances and the vehicle has now exceeded 220,000 miles. At 200,000 miles, the average bore wear of Commer engines is found by MAC. to be about .003 in.
[The transport operations of the Metal Agencies Co., Ltd., were described in The Commercial Motor on May 4 and May 111
At 354,123 miles, when the engine of a Commer 5-tonner operated by Messrs. Hopley's Haulag e, Stonnal, near Walsall, was decarbonized for the fifth time and standard rings were fitted, the maximum bore wear was .004 in. The engine Aas now exceeded 410,000 miles without major repairs.
Messrs. J. Kime and Son, hauliers, Lincoln, who run six 7-tonners and an articulated outfit, can beat even Hopley's record. One of the 7-tonners has passed 440,000 miles with the original engine, which required no replacement parts until it had covered 350,000. The bore wear was then still less than .003 in. New pistons, rings, valves, timing chain and a service crankshaft were fitted.
Serck Radiators, Ltd., Birmingham, have a 7-tonner which by the beginning of this year had covered 367,000 miles without major overhaul. A similar vehicle owned by W. Epton, Brothercroft, near Boston, has reached 380,000 miles. G. Hunter and Sons (Transport), Hull, Ltd., have another six, each with very high mileages.
Trailer Work Gives 6+ m.p.g.
To detail a list of other successful Commer operators would make a tedious catalogue, but the experience of John Rigby and Sons, Ltd., Bradford, deserves mention. One of their two underfloor-engined 7-tonners, during the first five years of its life, hauled a 5-ton trailer and yielded an average fuelconsumption rate of 61 m.p.g. It has now covered 240,000 miles. The other, running solo, has achieved 182,000 miles at 7.6 m.p.g.
Approximately this figure has been returned by a 7-ton tipper run by E. Clegg and Son, Ltd., Littleborough, which has "clocked " over. 180,000 miles and has been decarbonized only twice. New pistons were fitted at 90,000 miles. The vehicle is used locally and is nearly always loaded to capacity. J.C.G.