Cost is another important factor. Although toothed gears are relatively expensive to make compared to other means of mechanical transmission, they are inherently more precise, enabling lifting equipment designers to insert such components in the confidence that the design factors will remain practically constant over a long period of time. Accurate interaction of the moving elements with proper lubrication ensures low wear rates and a long gearbox life.
In lifting equipment the duties of gearboxes can be markedly different with mechanical hoists requiring high load and torque capacities, especially at start-up. Transit drives to produce a rolling action rather than moving a dead weight do not require such high capacity but require precision in operation as a main requirement in order to facilitate accurate and safe load handling.
Crane applications do not usually require very high speeds, but must offer durability over repeated use, plus a reversing facility, will usually be important. Depending on the nature of the crane or hoist installation, compact designs are often required to save on space and give the maximum lifting and travelling ranges within a building or other allocated space.
Lubrication
This last requirement means that gearboxes must be very economical in the use of internal space as well external dimensions. Repeated frequent use and little space can raise temperatures within the gearbox making lubrication and cooling important considerations. Some units are sealed for life whilst others may require the addition of oil, grease or a mixture of both as lubrication for efficient and assured operation. Whilst there is this trend to compact units and higher rated duties, there is also a demand for low maintenance. This can make conflicting demands on designers. Demag’s latest rope hoists have ‘lubricated for life’ bearings within a low-vibration, quiet-running drive. The Demag DR20, for example, which is rated at up to 50t load, is a compact unit with fast operating speeds. It utilises a 3-stage helical gearbox with ‘high endurance’ gearing and oil-filled, sealed, lubrication.
Simple spur gears, that is those with radial or ‘straight-cut’ teeth are the most common in general use, although they have been replaced by helical gears on most modern high-capacity lifting equipment. Helical gears have teeth that are not parallel to the axis of rotation. Thus the curved gear causes the tooth to form a segment of a helix. The teeth engage more gradually than do spur teeth with a bigger contact area, giving smoother and quieter running, whilst allowing better torque loads due to less shock loading of the teeth.
The gear arrangement also tends to be more tolerant in that the gear shafts do not need to be parallel, even when they are nominally parallel in the design. However, the better the parallelism of the teeth, the better the contact for transmission.
Use of simple helical gears result in a thrust along the axis of the gear, and this necessitates the use of thrust bearings to prevent undue wear and resist the sideways load. Higher sliding friction between the meshing teeth can be alleviated by special lubricants. A possible design solution to this problem is to employ double helical or ‘herringbone’ gears. The ‘mirrored’ gear cancels out excess axial thrust since the forces generated on one side are balanced by those on the other side of the gear.
A variation of helical gears often used in crane drives is bevel-helical gears. Forming a truncated cone, these gears interact in pairs set at angles between, but not including 0 and 180 degrees. Mitre gears are set at 90 degrees to each other.
Design flexibility
The design of a range of gearboxes on a modular concept enables hoist and crane manufacturers to select components that will both meet their design needs closely and save money by employing standard products, rather than having to commission a new design from scratch.
Except perhaps in larger and/or custom-designed gearboxes, the gearbox manufacturer will deal with the detailed design and manufacturing considerations such as accurate gear alignment and matching of the components. The applications designer can then select the gearbox according to the model best suiting the system’s power, speed and exterior alignment requirements. Although most applications will require speed reduction or same-speed gearbox duties due to the relatively high speed of electric drives, it may be possible to use the same gearboxes to increase rotary speeds depending on the selected input shaft. There may be more than one output shaft, if required (2-way, 3-way or 4-way designs), with the relative speeds dependent on the gearing.
In recent years there has been a trend towards more compact designs of gearbox. This feature can be very important in overhead crane design considering the often limited space available. Considerations include low headroom for hoists and the available space at the end of workshops for 90-degree changes in drive direction for transit transmission.
Standard ranges
Recently the US-headquartered Baldor group acquired the Dodge Quantis range of gearboxes, which are applicable to lifting applications. The UK company, based in Bristol, stocks all the necessary components, within the modular design concept, to assemble the gearboxes required. The Dodge Quantis range of gearboxes and gearmotors includes in-line helical (ILH) and right-angle helical bevel (RHB) speed reducers and gearmotors. The RHB designs are particularly useful in hoist and overhead crane applications in being able to direct the drive train through 90 degrees.
A spokesperson for Baldor in Bristol said: “Our assembly service enables OEMs, system integrators and system maintainers to obtain custom-configured power transmission products within days.”
PowerJacks of Fraserburgh, Scotland, has also made a recent acquisition in this product sector, having bought the NeeterDrive range, which it markets in association with its ranges of hydraulic screw jacks and small winches. NeeterDrive is claimed to be the UK’s largest manufacturer of spiral bevel gearboxes. A feature of the company’s website is a database of 2-D and 3-D CAD drawings to facilitate incorporation of the gearboxes and other products within customers’ designs.
There are three ranges of gearbox: Range-P, Range-N and Range-PowerGear including special variants as well as standard models. The Range-P is of a compact ‘monobloc’ design that is sealed for life and therefore maintenance-free. Shafts of hollow drives are used for 2-way, 3-way or 4-way transmissions with 1:1, 2:1 and 3:1 gear ratios. Power ratings are up to 7.35kW and torque capacity up to 50Nm. Manual disengaging and reversing facilities are available. The ultra-compact Range-N comes with motor adapters and gear ratios up to 4:1 and torque ratings up to 3000Nm. The heavier duty PowerGear models have gear ratios up to 5:1 and torque ratings up to 5200Nm.
PowerJacks supplies the NeeterDrive gearboxes internationally as single items or complete systems, with popular models available ex-stock.
Higher rating
Gearboxes of less compact design but with much higher ratings for reduction transmissions can be obtained by using helical gear designs. The PowerJack range includes helical worm gear and in-line helical designs. The former can handle power of up to 45kW without exceeding a maximum output torque capacity of 10kNm. Standard double reduction gearing gives ratios of up to 250:1, but up to quintuplet reduction is available. The in-line versions have power ratings up to 90kW and maximum output torque of 11kNm. Standard double reduction ratios are up to 70:1 but, again, quintuplet reduction is available. In both cases the drive shaft can be supplied with motors or linked to other transmission.
Specialists
In most cases specialist gear manufacturers supply a wide range of industries and applications rather than supply a narrow range such as lifting gear. As one example of several manufacturers in Asia, Ashoka International of New Delhi entered into gear manufacture to produce high-speed gears for its turbine division, but now makes designs for a broad range of applications. Precision gears are made to leading national and international standards.
Ashoka makes reduction gearboxes for crane and hoist applications with ratios up to 348:1 and power ratings up to 580hp (433kW). Other reduction gearboxes are made with helical gears and worm gears. Ashoka makes the point that the tooth profiles for its single- and double-helical gear units are precisely calculated for ideal shape. Longitudinal correction is made to compensate for any deflection and misalignment. The axial force generated by a gear unit is minimised through the use of combined journal and thrust bearings, or by using separate, self-contained, tilting pad thrust bearings. The tooth design and optimum contact, combined with a robust housing, guarantees low vibrations and noise levels.
Reduction gearboxes are also available using a planetary gear or epicyclic design (Able range) with power capacities up to 5.0kW and ratios between 1:3 and 1:81 from input speeds of up to 6000rpm. Planetary gear units offer the ability to carry high power and reduction ratios in a small space (high power density). Designs are usually complex and the bearing loads high.
Ashoka now supplies its products throughout Asia, Europe, Africa and Australasia.
Crane manufacturing
Also in India, materials handling equipment manufacturer Ganesh Engineering of Gujarat, employs components from manufacturers of international reputation such as Tata, Alstom, Crompton and Siemens. The company manufactures single- and double-girder interior and gantry/Goliath cranes with capacities of 1t–60t. All gearboxes used are of the helical type with mild steel fabricated housings. Pinions and output shafts are supported on roller or ball bearings to reduce friction. The gears themselves and pinions are precision-machined from En-24/En-19 materials and hardened to withstand heavy loads.
Gearboxes for wire-rope hoists with capacities of up to 10t are direct-mounted to the rope drum and driven by a motor with a geared coupling or geared brake drum. The gears are machined from En-9/En-8 materials. Ganesh also manufactures its own crane-duty gearboxes with cast-iron housing, and with gears designed for durability and long life.
Market considerations
The structure of the market for overhead crane and hoist gearboxes seems even more complicated than the technical considerations. A business performance portfolio analysis published by Plimsoll on the UK power transmission equipment sector studied 459 companies in this country alone. Whereas this study covers all types of power transmission, only a small proportion of which would be appropriate to overhead crane and hoist applications. It does, however, indicate the broad range of types of supplier that a purchaser may be able to choose from.
The major crane and hoist manufacturers will supply service and replacement of gearboxes associated with their products, whether or not they manufacture the gearboxes in house. There are also many other types of independent suppliers ranging from dealers in used equipment and parts, through specialist service contractors in maintenance and refurbishment to a few companies offering turnkey design and fabrication services for large cranes and gearboxes to suit specific requirements. For successful installation and operation it is clear that high level skills are required at both ends of the power transmission spectrum from the design and interaction of gears to the applications of the correct gearbox to the power, torque, dimensional and operation cycle requirements of the lifting equipment. There also needs to be good communication between these two areas of skill so that the limitations of a gearbox are understood by the designers and owners of the lifting equipment.
In order to minimise downtime of frequently used cranes manufacturers are placing an emphasis on speed of supply, either from stock for gearboxes widely used by manufacturers and operators, or just-in-time manufacturer delivery for regular orders. The alternative, which can sometimes impose an unacceptable burden on storage and financial resources, is for the crane owner to keep spare drives available on site.
Transmissions manufacturer Flender was acquired by Siemens in 2006, and is now know as Siemens Energy & Automation. The firms emphasises the aims of close customer contact and specialist advice for product development, short delivery times, fast availability and strict adherence to complete ‘on-time’ delivery orders. Amongst other transmission components the company manufactures helical gear units, bevel-helical gear units and planetary gear units. All three have been applied to travelling gear drives for overhead travelling cranes. Helical gearboxes are usually recommended for hoist installations. The Siemens range of helical gears has power ratings up to 4500kW and torque ratings up to 870kNm for transmission ratios up to 1:450. Flender has also supplied planetary and bevel gear units for hoist drives for overhead travelling cranes, container cranes and foundry cranes. Gear units for overhead travelling cranes have extended total load distances and torque ratings from 11kNM–109kNm. Complete drive units are supplied together with a Flender-Loher motor and fitted disc brake.
