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Som e Im portant Applications of the Process

W dokumencie The Chemical Age, Vol. 54, No. 1406 (Stron 23-29)

b y E . A . O L L A R D , A .R .C .S ., F .R .I .C ., F .I n s t.M ., a n d E . B . S M IT H bearings by electrodeposition, th e stopping- off on p arts w hich have to be hardened so

cadmium solution w orks satisfactorily a t room tem p eratu re, a zinc solution has to be worked w arm . F u rth erm o re, w hile a c a d ­ mium solution may be controlled analyti­

cally by the estim ation of cadmium and

Even g reater difficulty is experienced when articles have to be tre a te d in a plating

avail-C>34 THE CHEMI CAL AGE Ju n e 8 , 1 9 4 6

tice and is considerably cheaper.

The A .I.D . lnid down certain specifica­ thickness satisfactorily. Also, if too thick a deposit is made on threaded portions, the objectionable in connection w ith elec­

tric a l ap p aratu s, e.g., radio chassis.

I t became desirable, therefore, to find some method of treatin g the zinc- p lated p arts so th a t they would w ith­

stand the humid Pacific conditions.

It was found th a t zinc and zinc-

■chromate coating the exact com posi­

tion of which has not been entirely this process is satisfactorily carried o ut, an iridescent greenish-yellow film will be formed on the article. The

Ju n e 8 , 1 9 4 6 THE CHEMI CAL AGE 635 difficulty was experienced, since various G overnm ent departm ents insisted th a t the film should have some definite appearance specified by them selves. T his difficulty, countries dem anded finishes which would harm onise w ith the surroundings. T hus it appearance. It was, therefore, ultimately- decided to brass-plate the steel cartridge cases.

An enorm ous num ber of cartridge cases were required of various sizes and shapes, and for this reason it was desirable to install autom atic p lan t to deal w ith the output. A careful investigation was, therefore, made 011 the brass plating process and the condi­ of protection against corrosion. Kleciro- deposition, however, can be used for p ro ­ taking suitable precautions to ensure its adhesion, and satisfactory hardness and m echanical properties.

W hen a p a rt has to be built up it is first cleaned by any of the stan d ard m ethods and then stopped off so th a t only the portion to

636 THE CHEMI CAL AGE Ju n e 8 , 1 9 4 6

be treated is exposed to the solution. T his stopping-off process is usually accomplished by dipping the article concerned into a bath in sulphuric acid. A fter thorough swilling,

F ig . 2. S te e l d ie fa c e d w ith c h r o m iu m

taken ordinary plating work adopted iliis process and were able to salvage a large satisfactorily fo r plating heavy deposits. The building up of thick deposits, however, r e ­ of lubrication. A sm ooth, highlv-polished chrom ium surface does not hold oil well and back-etching the deposit a fter deposition.

F o r some reason, not fully explained, W hen

Ju n e S, 19.46 THE CHEMI CAL A GE 637

F ig . 4 . B e a r in g s e t u p in s p e c ia l v a t, w ith a n o d e r e a d y fo r d e ­

p o s itio n .

w ith the lead. T his alloy resists corrosion by m ost of the organic acids and other m aterial found in lubricating oil, and a b ear­

ing so formed gives extrem ely long life.

W hen the lead deposit itself becomes -worn nr corroded it can be removed and a fresh deposit m ade on th e original bearing, so th a t it is possible by this m eans to prolong the life of the bearing alm ost indefinitely.

Bearings of this type have been used in high- duty engines, and -in fact are alm ost essen­

tia l in the modern types of aeroplane engine.

Investigation of the conditions under which bearings operate have, as m entioned above, shown th a t there is little actual w ear.

F o r high-duty bearings, however, it is neces­

sary th a t th e bearing m etal should have a high h eat conductance so as to remove the heat formed on the surface, as this m ight a chrom ium deposit is etched it w ill usually

etch in the form of a num ber of fine cracks w hich ru n evenly over th e surface and en­

sure satisfactory lubrication. In some cases a surface of this type can be produced m echanically.

W hile for certain purposes, such as cylin­

d er bores, i t is desirable to have a very hard surface, for the bearings of crankshafts and sim ilar p a rts it has been found th a t a lead surface is satisfactory. The ordinary lead- bronze bearing requires to be carefully fitted and run in over a long period before the engine can be put into service. If, how­

ever, this bearing surface is faced w ith about 0 .0002 in. of lead it w ill be found th a t the running-in period can be dispensed w ith and th a t the bearing itself will last much longer and give a far b etter service (Figs. 4 and 5).

C areful investigation has shown th a t a good deal of w hat was previously considered to be bearing w ear is really caused by corrosion of the bearing surface by the lubrication.

In point of fact, the two m etal surfaces should never come into contact and if a satis­

factory film of lu b rican t is m aintained, in ' theory, there should be no abrasive w ear at all. In practice, it has beeh found th a t the lead surface, being som ewhat porous by n atu re, tends to m aintain a film of lubricant in the bearing and a t the same tim e resists corrosion by th e lu b rican t. T his resistance can be m aterially increased by infusing into

the su rface; the whole bearing is then heated a t 180° C. for about one hour, u nder which conditions the tin or indium alloys

F ig . 3 . S e c tio n of th e in s id e of a m o t o r - b u s c y lin d e r , s h o w in g s c o r in g of th e c h r o m iu m c o a tin g d u e to th e fa c t t h a t t h i s w a s of a n o n - p o r o u s n a tu r e .an d d id

n o t r e t a i n th e o il la y e r .

the lead a sm all amount of indium or tin.

T he lead film is first deposited on th e b ear­

ing from a fluoborate b ath , after which a sm all am ount of indium o r tin is applied to

G a s THE CHEMI CAL AGE J Ü N E 8 , 1 9 4 6 highly satisfactory results in service. AVhile at the present moment th eir application crack-detector. T his isj necessary as stresses are often set up in th e gear teeth through Infra-Red. Evaporators and Driers, p ub­

lished by K estner E vap o rato r and E n g in eer­

ing C o., L td ., 5 G rosvenor G ardens, S .W .l.

P a rtic u la rs are given of laboratory units for developm ent and sm all-scale production.

otherw ise vaporise the lu b rican t and cause

resulting compounds may engender a vicious attack on steel parts. Bronze gears have been successfully protected against the attack by a treatm en t with indium . The

Ju n e 8 , 1 9 4 6 THE CHEMI CAL AGE 639

W dokumencie The Chemical Age, Vol. 54, No. 1406 (Stron 23-29)

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