Today's Concentric Drives Encroach on Parallel Shaft Drives' Turf
 In
years past, high torque drive system applications were the exclusive
environment of parallel shaft drives. Project specifications were
typically written calling for concentric drives to 50 HP (37 kW)
and parallel shaft drives above 50 HP (37 kW). Today, however, concentric
drives are capable of handling torques as high as parallel shaft
drives, often at a substantial initial price savings over parallel
drives, and with much quicker delivery.
Technological advances in power transmission components and in
manufacturing have enabled concentric drive manufacturers to deliver
higher torque through smaller packages. These advances include:
- Robotic welding which produces optimized housing weldments
- Modern machining centers which ensure accurate boring and
positive alignment of internal drive components
- High grade steels which permit higher shaft stresses
- Improved bearings, made from cleaner alloys and advanced
manufacturing processes which provide increased load-carrying
capacity and longer life
- Carburized and ground gears which run cleaner with no wear
- Gear tooth lead and profile modifications which optimize
tooth loading
- Improved lubricants which reduce frictional losses
- Modern cooling devices which increase heat dissipation
Where concentric drives were once three or four times the size of
the motor that drove them, today that motor may be twice the size of the
gear drive.
Selection Criteria
The informed buyer should weigh features and benefits of both concentric
and parallel shaft drives before making a purchase decision on a high torque
application. Considerations should include:
- Availability
Concentric drives are often a stock item. Parallel shaft drives may
have lead-times of several months.
- Price
Where the torque ratings overlap, concentrics may offer savings of up to
15-25% on initial price. (See Table 1)
- Features and Benefits
Here you must compare specifics. For example, does your drive manufacturer
offer the same concentric drive features as desired on parallel shaft
drives? Key considerations include:
- Gearing -- Carburized and ground gears provide reliability
and long life, and run "clean" compared to
through-hardened gears.
- Bearings -- Tapered roller bearings stand up to thrust and side
loads better than ball bearings.
- Seal design -- Lip seals provide good life only if the general
design is sound. Ground seal journals lengthen seal life. A
purgeable grease cavity, outboard from the seal, protects the seal
from contaminating dust and particulates.
Viton® seal material
may be desirable in high temperature applications.
(Viton® is a
registered trademark of duPont Dow Elastomers.)
- Lube level monitoring -- The lube oil is the lifeblood of the gear
drive. As such, it is extremely important to be able to quickly and
accurately monitor the oil level and to check that the oil quality is
good. A dipstick/breather arrangement is desirable as it allows quick
and easy visual oil-level checks, and lets maintenance personnel
check the quality of the oil by smell and feel.
- Field Repair and Replacement Parts
Both concentric and parallel shaft drives have their pluses and minuses
as relates to field repair and replacements parts. For example, a
horizontally split parallel shaft drive allows for in-place repair, without
draining oil or removing elements. In some cases, parallel shaft gear life
can be extended simply by "flipping" the geartrain to operate on
a new flank. However, when spare parts are required, concentric drive
elements or a complete concentric drive are often available much quicker
than parallel drive parts. In conclusion, a well-designed and selected
concentric shaft drive can do everything a parallel shaft drive can do,
and at a better price and with quicker delivery. It pays to consider this
alternative, even for high-horsepower, high-torque applications.
Table One
Parallel Drive vs. Concentric Drive
Typical High-Torque Application Relative Cost Comparison
Heavy duty belt conveyor, 350 HP (260 kW), 1750 RPM motor (drive input shaft)
68 RPM, (drive output shaft), 16 hours per day, 100° F (38° C) ambient
| |
Parallel Shaft Drive |
Concentric Shaft Drive |
| Drive |
$24,655 |
$19,754 |
| Cooling Fan |
$ 1,309 |
$ 700 |
| Drilled Bedplate |
$ 3,385 |
$ 2,360 |
| Low-speed Shaft Coupling |
$ 2,136 |
$ 2,136 |
| Low-speed Shaft Coupling Guard |
$ 826 |
$ 826 |
| Fluid Coupling |
$12,053 |
$12,053 |
| Coupling Guard |
$ 1,287 |
$ 1,287 |
| High-speed Side Backstop |
$ 2,813 |
$ 2,751 |
| Total |
$48,464 |
$41,867 |
|
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