The band drive system . . .

The worm drive system . . .

The worm drive system is coupled to the band drive system via a smaller drive shaft.

The worm drive includes a precision clutch mechanism which keeps the gear precisely centered at all times.

The worm drive's worm block assembly keeps the worm precisely aligned and engaged into the gear.

The worm block assembly pivots about a pivot point located tangent to the worm and gear. The balance block balances the entire worm block assembly about the pivot point, maintaining precise alignment of the worm and gear regardless of telescope orientation.

Our unique Hybrid Band-Worm Drive Systems* are the culmination of years of development and refinement. Hybrid Band-Worm Drive Systems incorporate the best features of friction, chain and worm drives, but without their inherent limitations. Some notable design features are:

• Observatory Class Pointing and Tracking Performance with virtually zero backlash and extremely low periodic error of approximately 1 arc second.
• Alignment Mechanisms are incorporated for precisely aligning the band drive's disks so that the stainless steel bands track accurately and without run-out.
• Temperature Compensated Design Mechanisms are incorporated within the band tensioning mechanisms and within the worm drives to prevent changes in ambient temperature from affecting pointing and tracking accuracy.
• Ultra High Precision Clutch Mechanisms protect the drive systems, the instrument and the operator from harm should the telescope encounter any obstructions while moving.
• Immunity from Damage from grit and other contaminants which can easily damage other types of drive systems.
• Extreme Longevity since band drives, unlike friction drives, never "wear out" and since the forces upon and within the worm drives are extremely small.

*Patents Pending

Two Drive Systems in One . . .

Our Hybrid Band-Worm Drive system is really two drive systems in one. The drive system consists of a band drive system which is coupled to a worm drive system. The band drive provides either a 6:1 or 7:1 reduction between the telescope axis and the worm drive. The worm drive provides an additional 1080:1 reduction between the band drive and the motor. Thus the total reduction is either 6480:1 or 7560:1 between the telescope and the drive motor.

The Band Drive

The band drive consists of two precision turned aluminum disks of unequal diameter coupled together with a stainless steel band. The larger disk is attached directly to the telescope's primary drive shaft (the RA axis or the DEC axis), and the smaller disk is attached to a secondary drive shaft located on a mounting plate assembly. The two disks provide a reduction ratio of either 6:1 or 7:1 with zero backlash. This reduction ratio reduces all errors and loads within the worm drive component by a factor of 6 or 7. The result is dramatically improved pointing and tracking capability which is far superior to a conventional worm drive and the equal of a more expensive friction drive.

Unlike conventional friction drive systems, a band drive is totally immune to damage from contaminants such as grit or sand. Sand, dirt and even large insects have been caught in the ObservatoryScope 20" band drive without causing any damage or any observable effects on pointing or tracking. For example, a conventional 1/2 inch wide 5 thousandths inch thick automotive shim can be inserted, without causing damage, between the band and either disk. The observable tracking error caused by this automotive shim is only 1 arc second! Even more remarkable is that this tracking error is gradual because it takes several seconds for the shim to get "caught up" between the band and the disk as the telescope tracks at sidereal rate. Don't try this test on a conventional friction drive system — you probably will permanently damage either or both of the friction disks due to the inherently high engagement pressures within the friction drive!

The Worm Drive

Cutaway view of the Worm Drive

The worm drive features a stainless steel worm and an aircraft quality aluminum worm gear. The worm drive features a rather complex precision drive hub for supporting the worm and the gear. All surfaces within the drive hub must be machined concentric to the drive hub's central axis to better than one thousandth of an inch. The drive hub features precision centering and clutch mechanisms for the worm gear and for the drive plate which supports the worm block and the drive motor. These precision mechanisms, accurate to well under one thousandths of an inch, keep the gear centered relative to the hub's axis and keep the hub centered in the drive plate even if the gear is rotated within the clutch. These are important features since any gear decentering would cause pointing and tracking errors.

The worm drive also includes many design features which preserve the precise alignment of the worm relative to the gear. These features assure accurate continuous lapping of the worm to the gear. The worm and gear will gradually become somewhat more accurate over time as the telescope is used, eventually achieving the theoretical limiting accuracy which is related to the thickness and diameter of the gear and the pitch and diameter of the worm. Additionally, the worm is supported by class 7 bearings to maintain the precise alignment of the worm relative to the gear at the microscopic level.

The Mounting Plate Assembly

The mounting plate assembly uses a secondary drive shaft to couple the worm drive to the band drive, and features simple mechanisms for precisely adjusting the alignment of the band drive's disks and for tensioning the stainless steel band. The band tensioning mechanism is spring loaded to maintain a relatively constant band tension over a very large ambient temperature range. The band tensioning mechanism pivots at a right angle relative to the imaginary plane defined by the primary and secondary drive shafts. This choice of pivot location provides a mechanical form of temperature compensation which prevents ambient temperature changes from causing a slight rotational misalignment of the band drive's smaller disk relative to the larger disk. This rotational misalignment, were it allowed to occur, would cause pointing and tracking errors due to changes in ambient temperature.

Hybrid Band-Worm Drive Specifications

ObservatoryScope uses two different Hybrid Band-Worm Drive System designs depending on the aperture class of the telescope. One design is used on our 20" and 24" aperture telescopes, and the other design is used on our 32" and 36" telescopes. The design used on our larger telescopes basically is a scaled up version of the design used on our smaller telescopes. Specifications for each drive system design are shown in the following tables.