t 
670 MECHANICS. 
it will Hip off at one fide. It is not eafy to give any fa- 
tisfaflory explanation of this faff; nor of another, that 
if, by accident, one of the pulleys is (topped while the 
(trap is urg d round by the motion of the other, it in- 
ftantly flies. . ft' its pulley, unlefs the edge of the pulley 
fhould be mu in wider than the (trap. This property is a 
great recommendation of it for fome purpofes, fuch as 
thrafhing-mills, flour-dreffing machines, lathes, cotton- 
machines, See. where any thing accidentally flopping the 
machines would deftroy them if driven by wheel-work; 
but the. Itrap (lips round, and very foon comes off, fo as 
to avoid all further danger. Belts of girt web, fuch as are 
ufed for faddle-girts, are fometimes ufed inltead of leather 
ftraps, though thefe are undoubtedly preferable. The 
itrap (hould be dreffed to an equal thicknefs and breadth 
throughout, and the ends very neatly joined ; that is, of 
the fame thicknefs there as at every other part. It is fome¬ 
times done by (ewing, but the beft method is by gluing 
them together, with a glue compounded of Irifli glue, 
i-finglafs, aje-grounds, and boiled linfeed-oil. The two 
ends being tapered away and overlapped are united with 
this cement, and will be as flexible as any other part, but 
fo ftrong that it will tear to pieces in any part rarher than 
at the joint. A tool for equalizing the thicknefs and 
breadth of the ftraps for belts is delcribed and figured un¬ 
der the article Leather, vol. xii. p. 413, 4. They will 
by this means be rendered very correct ; for nothing can 
be more unpleafant in machinery than the joint and thick 
places in the endlefs ftraps jerking over the riggers, and 
caufing a violent drift upon the centres every time by the 
increafed tenfion of the itrap. 
A mechanic, in calculating any extenfive piece of ma¬ 
chinery which is to depend upon ftraps for the commu¬ 
nication of its motions, particulaily if they are of great 
length to convey their motion to a confiderable diftance, 
and have much (train upon them, (hould always confider 
that fuch machinery will lofe fome of its velocity ; that 
the wheels, which are turned by ftraps, will never make 
quite fo many revolutions as they ought to do from a cal¬ 
culation of their diameters. This is generally fuppofed 
to ari!e from the (trap dipping, in fome degree, upon the 
furface of the wdietls it pafles over; but we are inclined 
to fufpeft that it arifes from another caufe which has not 
been inveftigated, viz. the elafticity of the (trap: for in- 
ftance, fuppofe that the diftance between two wheels con¬ 
nected by a (trap is ten feet, and that the (train upon the 
Itrap is (uch as to Itretch or extend it two inches in that 
length on the fide which bears the (train (called by me¬ 
chanics the leading-fide') ; on the other, or returning-Jidc, 
there will be no (train, and therefore the ftrap will return 
to its original length. In fuch a cafe the wheel which is 
driven will lofe in its motion two inches in every ten feet, 
becaufe the ftrap gives out that quantity in leading to the 
wheel, but takes it up again in returning, as foon as the 
(train is removed from it. 
Mr. Nichollon has deferibed a fpinning-wheel for chil¬ 
dren at a charity-ichool, in which a large horizontal wheel, 
with a (lip of buff-leather glued on its upper furface near 
the outer edge, drove twelve fpindles, at which the fame 
number of children fat. The fpindles had each a fmall 
roller, likewife faced with leather, and were capable, by 
an eafy and inftantaneous motion, of being thrown in 
contact with the large wheel at pleafure; each child, 
therefore, could throw her own part of the apparatus 
into work, or cauie it to (top as often or as long as (lie 
pleafed. n 
Small machines are fometimes turned by a catgut baud, 
the ends of which are united by a (mall (teel hook and 
eye, tiie hooks being fattened at one end and the eyes at 
the other. They are made with tubes, (or the reception 
ol the ends ot the hand, which are tapped with a (crew 
within tide; and the band, being tapered and ferewed 
into the tube, holds very (ait. But, to prevent it draw¬ 
ing out, a (mall quantity of rolln (hould be applied to the 
end ot the band which projects through the tube, and a 
hot wire being touched to it finges and hardens the end, 
that it will never draw out of the tube. This method is 
conftantly ufed in fmall lathes, and works very neatly. 
The pulleys for a catgut band (hould always be cut with 
a (harp angular groove, for the reception of the band ; and 
it fhould not touch the bottom of it, or it will be liable 
to flip. For the fame reafon, the pulleys are beft made 
of wood, becaufe metals foon acquire a polifli, which 
prevents the band holding firmly upon it. The wood 
(hould be cut with its grain acrofs the direffion of the 
band, that every part of the circumference may be of a 
fimilar texture. 
Endlefs chains are fometimes ufed to communicate motion 
of wheels; and frequently cogs are formed on the wheels 
to be received into the links of the chains. This method 
is very practicable on particular occafions ; and, though 
it has not advantages to put it in competition with cog¬ 
wheels adding upon each other when they can be applied, 
it is in many inftances a valuable refource to the engineer 
to convey motion to fome diftance when it requires to ba 
accurate, and where it would injure the operation of the 
machine if any motion was loft by the dipping of bands. 
In making fuch chains, the greateft care is neceffary to 
have all the links precilely of one length, and the cogs 
very accurately fitted to them, or a great friftion will be 
caufed by the cogs forcing themfelves into (paces not ex¬ 
actly fituated to receive them. The beft way is to make 
the links in the manner of watch or clock chains, with 
iron plates, and holes drilled through them at equal dis¬ 
tances, to receive crofs pins upon which the cogs are to 
aft. By this means the lengths may be made far more 
accurately than by bending the iron in the manner of 
common chain-links. 
The conftruftion of bearings, pivots, gudgeons, or 
centres, of fpindles, as they are indifferently termed, is 
a mod important point; thefe parts being the principal 
feats of that friction which is the deftruction of all ma¬ 
chinery. Pivots are always made of iron or (teel, both 
becaufe thefe fubftances are better adapted for rubbing 
furfaces, and that their (trength admits the pivot being 
as fmall as poflible ; the bearing, or bed to receive the 
gudgeons or pivots, fhould be of a fofter metal, as brafs, 
tin, or zinc, and kept well fupplied with oil when at work. 
Hardened (teel is a molt admirable fubftance for pivots- 
which have a great (train to bear, and a rapid motion. 
The bearing or bed may alfo be made of the fame mate¬ 
rial ; and is the only inltance where two bodies, having 
friftion againft each other, can with propriety be made of 
the fame fubftance: for it is found, that, where iron or 
foft-fteel furfaces are worked with a friftion againlt parts 
of the fame fubftances, the friftion and abraiion are far 
greater than when a fofter material, as brafs, tin, hard 
wood, ivory, horn, &c. is ufed. The great difficulty of 
making hard-fteel pivots to fpindles is the only reafon 
they are not generally ufed; but there are fome cafes, in 
which nothing elfe can be employed: where fteadinels 
and accuracy of motion are required, and great velocity 
at the fame time. To obtain this accuracy, it is neceffary 
that the pivot (hould be fitted, and kept in accurate con- 
taft, with the interior furface of its focket or pivot-hole; 
and this will prefent a (iifiicient accefs of oil, to prevent 
any other fpindle than one of hardened (teel from burn¬ 
ing or heating by th.e friftion, when in rapid motion; and 
the expanfion occafioried by this heat increafes the preffure 
and the friftion, till the pivot becomes fixed in its focket, 
and will rather twift off than turn round in it. The fpindle 
for a turning-lathe mult always be of hard (teel ; and, even 
then, a failure of fupply of oil for a moment will caufs 
it to burn into the collar. Circular faw-fpindles are fre¬ 
quently burnt in the lame manner ; their motion being 
very quick. 
The beft form of a gudgeon or pivot for a fpindle is, 
that of a cylinder, with a flat Ihoulder, to prevent it from 
drifting its pofition endways. This form will bear moil 
(airly and lleadily; but it is neceffary that the focket, or 
brafs 
