Steam-engine design. For the use of mechanical engineers, students, and draughtsmen by Whitham Jay M. (Jay Manuel) b. 1853

Author:Whitham, Jay M. (Jay Manuel), b. 1853
Language: eng
Format: epub
Tags: Steam-engines
Publisher: New York, J. Wiley and sons
Published: 1889-03-25T05:00:00+00:00

1^ zixi zixnl r«:-Fer

r-in.^ VI

"•I "IT m»±« ss -•::i^ as

pressure remaining after correcting that on the piston, P, for the friction of the cross-head guides.

a = the radius of the wrist-pin in inches; / = length of the connecting-rod.

The moment of friction in inch-pounds is P^fa. This resisting moment, transferred to the crank-pin, is equal to that of a force

— ^ — acting in the direction Ee. The tangential component of

this resistance is * cos (0 + B), The value of cos {Q + 0)

is one when the engine is on either dead centre, so that the eflfect of the wrist-pin resistance is greatest at the ends of the stroke. Let a = 5 in., / = 8 ft., / = 0.05, and cos (6^ + 0) = IJ then the greatest tangential resistance is

5 X 0.0 5/^, ^^o^p -^-^- = o.oo26P„

or 0.0026 lb. for every pound pressure on the piston. This decreases until the connecting-rod becomes tangent to the crank orbit, when it disappears, giving a curve similar to vmm in Fig. 119.

72. Friction of the Crank-pin and Crank-shaft Journals. —We will call the piston pressure, after deducting the resistance due to the friction of the wrist-pin, P^.

The resistances at the crank-pin and crank-shaft may be considered at the same time. The pressure on the crank-pin

p

in the direction of LE^ Fig. 115, is ^—\ and the pressure on

cos q>

the shaft in the direction EO is equal and opposite, and is the

other force for the turning couple.

Let b = the radius of the crank-pin, c = the radius of the

shaft, and r = the length of the crank. The moment of their

combined friction about their respective axes is

cos 0

and the tangential resistance is

r cos 0

STEAM-ENGINE DESIGN.

If we neglect the angularity of the connecting-rod, or a^ume

the crank-pins to be at either dead-point, 0 = o, and cos <f> ^ \.

OE The crank-effort, neglecting friction, is P, X J^fjt «"id the cffi-

£iency of the crank for any one position is

E =

P OJ[_pf±±£'' ■^' OE *J r cos 0

p.y.

OH OE

OH f^_±_c OE ^ r

^ = O

OH OE

OH

But -yyp is sin B when 0 = o; hence

E =

sin e-Ub^ c)

sin 6

r sin o'

If the values b and c are equal, as is frequently the case,

r sin d When the engine is on its centre, ^ = o, and

-£ = I — 00 = —.00.

The efficiency remains negative for a certain angle corresponding to £ = o, and then passes to a positive value. To find this angle we proceed as follows:

o= I

sin B =

r sin 61 *

^,.-„-.|/<Hil)

B = sin

|/ie±£)|

Xet 3 = r = 6 in.,/= 0.05, r = 24 in. Then B = sin "' 0.025 = 1° 30' (nearly).

Inside this angle no pressure, however great, could cause the engine to start, and even for some distance beyond this the work absorbed in friction is greater than that utilized. For a slow-moving engine of short stroke the efficiency could be much decreased, as the frictional resistance is directly proportional to the diameter of the journals and inversely proportional to the length of the crank.

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