to the type described above, but use a driven elastomeric traction tire pressing
on the underside of the runway beams.
Portal and Gantry Travel Drives. Travel drives on portal cranes,
container cranes, gantry cranes are mounted directly on the travel truck
structural frames. Each drive is self-contained with an electric motor, brake,
and speed reducing gearing.
The older portal and gantry cranes use a combination of a gear reducer
and a number of open gear sets for speed reduction. Their final drive is comprised
of an idler gear driving two wheel gears which are either bolted directly to the
wheel or pressed and keyed on the wheel axle. With this arrangement, the crane
travels on a complement of powered trucks (in which both wheels are driven) and
idler trucks (in which neither wheel is driven). The interlocked set of idler
gear, two wheel gears and their wheels, and the rail present design (component
sizing) and machining (accuracy) complications. Since both wheels are forced to
rotate at identical speeds, their tread diameters must be perfectly matched to
preclude locked-in torque and stress from building up. In practical terms, the
two driven wheels can never have identical tread diameters, and will attempt to
travel slightly different distances along the rail for the same amount of
rotation. The only means of compensating for this difference is for one or both
wheel treads to slip on the rail surface. The imposed wheel slippage torque due
to high wheel loads and friction coefficients dictates the design parameters for
many of the mechanical, structural, and electrical components of the travel
trucks. The open gear sets present difficulties in shielding them from the
weather and wear-causing contamination. Hand application of grease provides poor
Newer cranes avoid the wheel slippage condition by driving only one wheel
per travel truck, and thus making all travel trucks identical in this sense.
Furthermore, all speed reduction is achieved within gear reducers (which are
sealed and oil-bath lubricated). The entire drive assembly is normally a flange
connected set of electric motor, gear reducer, and disc brake. The gear reducer
has a hollow shaft which drives the wheel axle through a spline, press fit and
key, or a compression sleeve. The drive assembly may be bolted to a structural
foundation on the truck frame or supported by the wheel axle and a torque arm.
With either mounting, the drive assembly is removable as a unit for maintenance or
replacement. The preferred gear reducers are parallel shaft helical and right
angle spiral bevel types with all gears arranged horizontally; other speed
reduction units and vertical gear arrangements may be used with the approval of
Wall and Semi-Gantry Cranes. Travel drives are combinations and
modifications of A-1, A-4, or the newer designs of the portal and gantry cranes.
Trolley Drives. Trolley drive wheels are mounted on a common axle,
extending across the width of the trolley frame, which is driven through a gear
reducer mounted anywhere between the wheels or outboard of one of them. The gear
reducers may be parallel shaft or right angle type with spiral bevel or worm
gearing. It is desirable to avoid vertical gear trains to preclude the
possibility of oil leakage around the shaft seals.
Trolley drives which are integral with the hoist/trolley units are
standard commercial designs similar to those described in paragraph 126.96.36.199 above.