Power Transmission by Belt: Part II
Power Transmission by Belt: Part II
By George B. Loughery
 Editor's note: This is the second of a two-part series on belt power
transmission.
Continuing our discussion of belt power transmission, we now apply
the 18 considerations for a successful flat belt drive that were
developed in “Power Transmission by Belt: Part I,” Gas Engine Magazine,
April 2005.
Lets assume we want to run a 10 kilowatt, 1,800 RPM DC generator
with our antique 30 HP tractor. We are told we'll need about 17 HP to
power the generator at the full 10-kilowatt output. The tractor's belt
pulley capability will undoubtedly be sufficient since this rating is
always less than the engine's rating by just a few horsepower. This is
because of the slight amount of loss attributable to gears and
bearings. The rated speed of the belt pulley is 1,000 RPM. The pulley
is 10 inches in diameter and 7 inches wide.
Some time ago, we mounted a generator in a shed with a hinged door
on the side for a belt to enter. We placed the generator in such a
position that the following criteria were met: 1) the pulley-end of the
generator will be in a people-safe area; 2) the tension side of the
belt will be on the bottom when the generator is being turned in the
proper direction; 3) the tractor can easily be maneuvered and placed so
that the belt length will fall into an acceptable range. Ten feet is
about all the pulley-to-pulley separation we can get in this case.
Certainly not very far, but it will be okay.
First we'll check belt speed. The tractor pulley may have to be
changed if the belt speed is not in our desired range at the rated
speed of 1,000 RPM. Factory installed pulleys are normally sized to
give belt speeds in the correct range. Belt speed is calculated by
multiplying pi (3.14) by the diameter of the pulley by the RPM. In our
case the belt speed is 31,400 IPM or about 2,600 FPM. This is good.
Since the RPM of the generator shaft must be 1.8 times the tractor
shaft, the load pulley must be smaller than the PTO pulley by a factor
of 1/1.8.
Cross-multiplying and dividing both sides by 1,800 gives
approximately 5-1/2 inches for the generator pulley diameter.
A 5-1/2-inch pulley would be nice, but most likely a 5- or 6-inch
pulley will be located. Either one could be used, but the following
concerns must be evaluated: The larger pulley is better as far as power
transmission is concerned, but the tractor pulley will now need to run
at 1,080 RPM. If 5 inches is used the tractor pulley can run at 900
RPM. The question in this latter case is – can the tractor supply 17 HP
at this reduced speed?
Two minor correction factors must be figured in at this point.
Recall that we will have about 1 percent creep. And the overall
efficiency of the belt drive will be around 95 percent. Taking these
into account, we must increase tractor speed by about 10 RPM to account
for creep and we must add about 5 percent to the power that the tractor
must supply to account for inefficiency, thus about 17-3/4 HP total.
So, if we use a 5-inch pulley the tractor must run at 909 RPM and
with a 6-inch pulley tractor RPM must be about 1,090. It must provide
17-3/4 HP in either case.
Now, what belt width do we need? If we use the 3 HP per-inch figure,
we need a 6-inch belt. The tractor's 7-inch wide pulley is fine and we
need to be sure whichever generator pulley we decide to use is about 7
inches wide.
If we don't find an pre-made belt, we cut a piece and properly
square the ends and join them either by gluing or by using the
wire-type lacing hooks and rawhide pin, as described in Part 1.
Take time to properly align the pulleys. A 3- or 4-foot straight
stick placed against the side of one pulley will be a big help in
aiming the side of one pulley to the corresponding side of the opposing
pulley. If this is done on both pulleys, you can be sure the pulleys
are not only lined up side-to-side but the shafts are parallel. Put the
belt on, smooth-side down, and with the feather edge, if glued, facing
away from the direction of travel. Initial tension, per side, should be
around 40 pounds per inch of width. The tractor is aligned and chocked
so that it exerts about 480 pounds pull on the generator
After the belt is properly adjusted and checked, pull the belt by
hand to be sure that it is tracking properly. The tractor is started
and the generator is slowly starting to rotate. The tractor's throttle
is adjusted so that the generator runs at 1,800 RPM, with the expected
electrical load applied. Once this is set, the tractor's governor
should hold the generator speed fairly constant even with somewhat
varying load. Electrical/electronic regulators are available to
maintain very close control of output voltage. Hand-held tachometers
are widely available and should be used as often as required to check
speeds.
If an AC generator is being considered, a frequency meter connected
to the electrical output will serve as a tachometer. You will find that
frequency control is difficult and the frequency can be expected to
wander by a few hertz from time-to-time. If you plan to use an AC
generator be sure to check on the RPM requirement! Here again,
regulators are available to control the voltage to a very close
tolerance.
V-belts
Unlike flat belts, these belts make use of their angled sides to
wedge into the groove in the pulley or sheave (pronounced shiv). Rumor
has it the word “sheave” does not have an equivalent, in other major
languages, and translation difficulties may force the word “pulley”
into favor.
The wedging action can be visualized best by flexing an actual belt
and observing that as the belt bends, as if going into a sheave, the
angle changes as the top is stretched and the bottom compressed. This
makes the belt sides steeper, i.e., less angle. As the belt enters the
sheave groove the belt is immediately restrained and the angle cannot
change, but the effect is to grip the sides of the belt very tightly.
V-belt drives can operate with tight-side to slack-side ratios around
5-to-1, i.e., T2 = 1/5(T1). They can handle high horsepower for their
size.
Main Types of V-belts:
1. Light Duty
(sometimes called fractional horsepower): The nominal angle of the
groove in the sheave is the same for the whole series (38 degrees), but
must be decreased as the diameter decreases because of the significant
change in angle as the belt is bent around the small diameter sheaves.
The smallest sheaves allowable require angles of about 30 degrees.
These sizes and angles are published in the literature, e.g.,
Machinery's Handbook. The horsepower ratings of the 4L and 5L series is
about 2 HP with sheaves not smaller than about 4 inches and belt speeds
around 4,000 FPM.
2. Multiple or Industrial V-belts: The name is misleading. They can be
used singly but they are made to closer tolerances so that they can be
used in sets if desired. For critical applications they should be
ordered in matched sets. This type is measured in pitch length, not
outside length like the light-duty type.
The nominal sheave angle is 38 degrees, decreasing to about 34
degrees for the minimum pitch diameters shown. Horsepower ratings of
this type reach into the hundreds for the larger sizes. To avoid
confusion, I like to refer to these as the “letter type.”
3. SAE Standard
V-belts: These are used in automotive applications and are covered in
the literature. Widths are 3/8-inch, 1/2-inch, 11/16-inch, 3/4-inch,
7/8-inch and 1-inch. The nominal sheave angle is also 38 degrees,
decreasing with small sheaves.
4. High Profile
Wedge V-belts: These belts have a taller cross-section than all other
belt types and have somewhat higher horsepower ratings because of the
greater side contact area. They go by the designations 3V, 5V and 8V,
and are 3/8-inch, 5/8-inch and 1-inch wide respectively. They also have
a nominal groove angle of 38 degrees for sheaves in the range of about
6-12 inches. This is reduced for smaller diameters as with the light
duty and “letter type” belts, but must be increased to as much as 42
degrees for the larger diameters. In addition, the groove depth may
need to be increased in a given sheave to use these belts. All these
dimensions should be studied in the manufacturer's literature before
any usage.
It is my recommendation that unless a need exists for the highest
possible horsepower capacity or for replacement, that light duty or
“letter type” (multiple) belts be used, depending on expected load.
5.
Double-sided V-belts: These belts are for applications where the belt
must drive on the top as well as the bottom. It contacts some pulleys
with one side and others with the other side. The net effect is
reversal of direction of some pulleys. The belt resembles two regular
v-belts glued back-to-back. These are often used on farm equipment and
serve well, but they can't be used on small pulleys or where reverse
rotation is required on shafts that are close together. The height of
these belts makes them susceptible to damage from excess flexing. One
popular type is designated AA, BB, CC, etc.
6. Detachable Link V-belts: This is another type of belt that is kind of a
do-it-yourself belt, where you assemble individual links to form a belt
of any length. These belts perform about as well as the regular v-belts
and have the major advantage of being able to be installed in cases
where endless belts can't be used. They are available in 3/8- to
7/8-inch widths.
7. Modern Thin Flat Belts: In the 1950s, a new variety of very thin,
toothed flat belt commonly known as Gilmer belts appeared. They could
handle high tensile loads at high speeds and around small pulleys. Over
the years these have been improved and 100 HP per inch is common.
Speeds range up to 20,000 FPS. These belts are used as final drive
belts on high-powered motorcycles and other heavy-use applications.
These belts have teeth that engage grooves in the pulley, which means
there is zero creep and the relative position of the pulleys does not
change. These belts typically have fiberglass, Kevlar or steel strength
members and stretch is virtually eliminated. This makes alignment
somewhat critical. They are about 99 percent efficient because of the
small amount of bending and are available as thin as 1/32-inch. They
are extensively used in office equipment and for automobile timing
belts.
Ribbed belts are another variety of thin flat belts and are used
commonly for vehicle fan belts. They are often used in a serpentine
configuration. They have a series of small v-belt-like projections that
run in corresponding grooves in the pulleys. In automotive applications
these belts handle up to about 10 HP at speeds well above 10,000 FPM.
General V-belt Considerations
All v-belt series are available in the notched design. These belts
have the same horsepower ratings as the standard belts and are
particularly useful with small sheaves.
The force required to deflect the belt 1/64-inch per inch of span
usually judges v-belt tension. This is measured at the center of the
span. These forces are in the neighborhood of 4 pounds for size A, 5
for B, 12 for C and 25 for D. For more precise tension data consult the
manufacturer's specifications. New belts are set up a bit tighter than
the desirable final tension because they quickly wear-in. Tension is
not too critical, as long as there is enough so that no slip occurs at
the full load expected.
If the load is intermittent and/or highly inertial, chirping may
occur. This is okay, but squealing is not and must be corrected
immediately.
Oil and grease contamination is very detrimental to a v-belt drive
and must be prevented. Belt dressing should be avoided unless extremely
dusty conditions exist. Creep occurs to about the same extent as the
flat belt drives, maybe 1 percent. Steel reinforced belts exhibit less
creep. Idlers should be used only if absolutely necessary. Back-side
idlers are particularly bad. The problem arises from the fact that the
belts are thick and the extra flexing causes premature cracking. If an
idler must be used it should be as large a diameter as possible.
Special thanks to George B. Loughery and the Hay Creek Valley
Historical Assn. for permission to publish this article, which
originally appeared in the Association's Late Summer 1994 issue of The
Journal.
For a complete list of references used for this article, please
visit the Gas Engine Magazine website at: www.gasenginemagazine.com
Contact George Loughery at: g19605@hotmail.com
Contact the Hay Creek Valley Historical Assn. at: www.haycreek.org
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