Gearing is one of the most important components between prime movers and
driven units. If gearing is not selected properly, it can cause many problems.
Gearing transmits great power at high rotational speeds. Recent advances in
turbomachinery technology, especially in turbines, compressors, couplings, and
bearings, have required gearing to withstand high external forces. To design
problem-free equipment, it is important to consider the effect of the external
system on gearing. Thus, all the factors that influence design, application, and
operation of gear drives should be considered in the design phase.
Since problems encountered with gears are complex, it is unfair to blame
them on the gear manufacturers alone. The gear supplier is much less informed
about the package than any other group. Problems should be handled as a team
effort between manufacturers and users. One factor causing problems is that the
system is not timed in terms of spring constants and masses. The gear is usually
the only item required to operate with metal parts in such close contact with other
components. This setup can result in early failure. Gearing is also subjected to
cyclic loading varying from 0 to 55,000 cycles per minute.
With current materials and heat-treating techniques, the use of high-hardness
gearing with tooth loads of 1500–2000 pounds per inch of face at pitchline velocities
of 20,000–30,000 feet per minute (6096–9144 mpm) is not at all uncommon.
In turbine-driven test equipment, gear drives have been built with pitchline velocities
as high as 55,000 feet per minute (16,764 mpm) and rotational speeds
approaching 100,000 rpm. The magnitude of internal forces and material stresses
coupled with the high speeds has resulted in gear drives that are dynamically
complicated and sensitive to influences from other components in the system.
The system characteristics of the entire train must be known so that the selection
of the gear will be proper. The major points affecting the system are:
(1) couplings, (2) vibration, (3) operation conditions, (4) thrust loads, and
(5) mounting type.
Couplings are a constant source of unbalance vibration, and critical speed
changes can be attributed to spacer shift and wear. Coupling lock keys can also
cause severe housing vibration, while shaft vibration can remain low. Therefore,
it is important to monitor vibration with accelerometers in addition to proximity
probes. Gear failure from high vibration is common when the gear and pinion
teeth operate within a few hundred microinches of each other. Accelerometers
can also monitor gear mesh frequencies and thus act as early warning devices.
Operating conditions must be known in detail.
In many cases, the gear manufacturer is provided with only the design horsepower
of the machine. Actual transmitted loads can be much higher due to the
proximity of torsional or lateral critical speeds. Surge in centrifugal compressors
can cause severe overload and result in failures.
External thrust loads are another major problem, and in many cases they lead
to double-helical gear selection. The gear housing and the mounting type of the
gear train are very important considerations in the overall life of the unit, since
improper mounting and expansion of the gear housing can lead to misalignment
problems.
A substantial structure to support the gear drive weight, thrust, and torque
reactions with minimum load deflections must be provided. At least two dowels
for locating each gear housing are required, and it is necessary to minimize housing
vibration from whatever source. Ideally, the structures should be reinforced
concrete of steel filled with grout. The inclusion of oil reservoirs in the structure
supporting major train components should be avoided, since unavoidable
thermal changes will have adverse effects on alignment. If a reinforced concrete
or a filled structure cannot be provided, resonance due to train component mass
and structure stiffness at system rotational frequencies or harmonies should be
avoided.
Gear Types
The choice between single- and double-helical gearing is sometimes difficult.
Both gearing types can be made to equal limits of accuracy as control of the
gearing accuracy is only a function of the accuracy and maintenance of the geargenerating
machines, machining techniques, and operator skill. A hobbed gear
is generated in a continuous process by a simple and easily maintained rack
from cutter, which will produce gearing of extremely high-profile accuracy with
virtually immeasurable spacing errors and uniform lead. Where both helices of a
double-helical gear are cut at the same time, or sequentially without a change in
setup, apex position error will be virtually unobservable in operation, and axial
vibration excitation from the mesh will be negligible. The same basic equipment
can be used for generating either single- or double-helical gears, although the
continuous finishing processes used for double-helical gears produce a higher
order of accuracy of lead and tooth spacing than does grinding if it is used for
finishing a single-helical type.
Thrust loading is a significant problem in the design of any gear unit, and
effects differ based on the choice of single- or double-helical gearing. In either
case an accurate estimate of the thrust loading is required to make an intelligent
compensation for it. With double-helical gearing, continuous axial loading can
be accommodated by a slight increase in capacity to account for the helix load
imbalance. API 613 and 617 specifications require that single-helical gears must
have a thrust bearing. It is also recommended (but not required) that doublehelical
gears have thrust bearings.
The increase in cost and the reduction of efficiency thus caused is only a
fraction of that incurred when a large-diameter, high-velocity thrust bearing must
be mounted on a single-helical pinion shaft.
Intermittent loading, such as that from gear couplings mounted on thermally
expanding shafting, is a different problem. This problem is accommodated in
a double-helical unit by judicious selection of helix angle and coupling size so
that axial coupling forces resulting from the transmission of torque are less than
the thrust force produced by each helix of the gear. This design assures that the
coupling will slip to relieve any axial loading and that the power balance on the
two helices will be maintained.
High-speed gear couplings are selected in which the pitch diameter is substantially
smaller than that of the pinion. Axial forces produced will be high, and the
combination should be examined very closely as a potential trouble source.
Double-helical speed gear is the first choice in calculating the accuracy of
loading and smoothness of operation. Predictable performance renders unnecessary
any deviations from easily defined and measured geometry. These gear
sets will be more efficient and have unmatched reliability if properly applied.
Reasonable intelligence must be exercised in coupling selection. This discretion
will assure vibration and noise levels that are often indistinguishable from that
of the connected machinery.
In single-helical gearing, all forces externally generated must be added to
the thrust produced by the gear itself, and the total is used on each shaft to
select the high-speed shaft thrust bearing. An error in thrust or bearing capacity
estimation will result in frequent failures of the thrust bearing or the associated
shafting. Single-helical gearing, due to asymmetrical loading from the helix,
has two sources of design difficulty which do not exist in double-helical gear
sets. The effective center of tooth pressure oscillates back and forth across the
face, putting substantial alternating loads on the shaft bearings. This oscillation
results in peak bearing loads substantially greater than those calculated, which can lead to early bearing failure. In addition, the helix-induced thrust force causes
the gearing to try to skew in the housing, unbalancing the bearing loading and
forcing the gearing to run out of parallel. Crowning of single-helical gearing is
used to counter the effects of shaft misalignment.