Differentially Pumped Rotary Platforms (DPRP / DPRF)

Overview

When it becomes necessary to introduce rotary motion into an ultra-high vacuum (UHV) vessel while simultaneously providing access for electrical, cooling and auxiliary motions, the differentially pumped rotary platform is an ideal product.

All DPRP units include as standard a worm gear for fine adjustment and an included 0.1° scale for measurement of angular precision and for adjustment. Both manual and motorized versions are available for purchase. The units may be purchased in either "Zero Length" or "Half Nipple" style flanges on either side.

This DPRP unit also incorporates the option for cross over pumping manifold. This option is strongly recommended in order to "rough out" the intermediate volume before starting the High Vacuum pump.

The DPRP unit may also be baked but should never exceed 150°C in order to maintain the integrity of all seals.

How a Differentially-Pumped Rotary Seal Works

The graphic to the right shows a cross section of a typical, differentially-pumped rotating seal.

Fluorocarbon seals will easily withstand a pressure differential of 1 X 10^-7 torr when properly fitted. Differential pumping can extend the range of a single seal significantly to maintain true UHV pressures. For example, in the graphic to the right, providing rough pumping (~10^-3 torr) in the Outer pumpout volume and moderate pumping (small ion or turbo pump) on the Inner pumpout volume, results in a pressure differential between UHV and the Inner volume of 10^-5 torr but virtually no "real" pressure differential to drive gas molecules across the seal. The result is a leak-tight, yet rotating, seal.

Rotary Concept

The DPRP may be rotated continuously in either direction. The direction of rotation may be reversed at any time. Small pressure bursts may be observed near the DPRP as it is rotated. This is normal due to the surface friction between stainless steel and the seal. Mass spectrometer studies show the bursts to be mostly water, methane, CO and CO₂ with small ammounts of H₂.

When pumping on both pumping ports, the typical pressure burst observed, with a system base pressure of 2 X 10^-10 Torr, will be 0.5 to 1.0 decade. Typical system base pressure recovery times range from 5 to 50 seconds, depending upon system condition and pumps used.

Linear and lateral alignment is maintained by a special ball bearing made of hardened carbon steel. These bearings will corrode therefore not allow moisture to condense on or in the seal. Do not allow LN₂ to chill the seal. To prevent moisture, dust and/or debris from entering the bearing, there is a fourth seal to protect it. (This feature is unique to differentially pumped rotary platforms.)

Installation

The DPRP is shipped clean and ready to install. Remove the protective polyethylene bags and flange covers. Be certain no foreign residue or particles adhere to the device or the sealing flange knife edges. As a precaution, it is recommended the flanges be wiped with a lint free cloth prior to installation. Install the DPRP on your chamber/manipulator/instrument as you would any other Conflat type flange. Use standard, 0.080" thick, flat copper gaskets. For the tapped flanges, use the bolt sizes recommended in the following table for installation.

Double check that your mating equipment has standard thickness flanges. These recommended bolt lengths are for standard thickness Conflat type flanges. USING INCORRECT LENGTH BOLTS FOR INSTALLATION CAN DAMAGE THE DPRF ASSEMBLY. The bolts may be supplied by you, or you may use a DPRF bolt set available from KJLC.

Cross Over Pumping Option

This option will include a cross over valve with the delivered product. This valve allows the user to first 'rough' out both the inner and outer stage with the same rough pump. Once the inner stage is at low enough pressure, the user can then close the valve to isolate the inner and outer stages from one another. The High Vacuum pump connected to the inner stage may be started.

Suggested Pumping

OUTER stage: >1.6 CFM at <10 microns

KJLC recommends the KJLC-RV206 pump for the outer stage pumping.

Connect a roughing pump to the OUTER port flange. The roughing system must have a pumping speed of 1 CFM, or greater, measured at the port flange, and be capable of achieving a vacuum base pressure of at least 10 microns, measured at the DPRP pumping port flange. The platform should be protected from backstreaming oil or other contaminants with suitable traps.

INNER stage: ~2 l/s at ~10^-6 Torr

KJLC recommends the 3S-CV-1V-5K-N-N Gamma Ion Pump for the inner stage pumping.

Connect a high vacuum pump to the INNER port flange. The high vacuum system should have a pumping speed of ~2 l/s or greater, measured at the INNER flange, and be capable of achieving an operating pressure of at least 5 X 10^-6 Torr, measured at the DPRP pumping port flange. Higher pumping speeds and lower operating pressures will decrease the effective leak rates and decrease the pressure bursts upon rotation. Care must be taken to insure that significant pump speed losses due to conductance restrictions do not occur by using long lengths of small diameter tubing to connect suitable pumps. This is the most common cause of poor performance upon initial installation.

Using One Stage Pumping Only

If you wish to use only one pump stage, connect the pump to the INNER port. Cap off the OUTER port or plumb the OUTER port into the INNER port but DO NOT seal the OUTER port. Using one pump or no pump on the DPRP may cause the system base pressure to degrade and limit ultimate system base pressure but will not harm the DPRP.

Configuration Options

This chart is to help you review all the available options for the DPRP. Please see below for an interactive product builder to assemble the options into a part number ready for order.

Ordering Code Example: DPRP0400-ZMHT-Y-HL

Gearing Principal

The fine adjust worm has bearings at both ends, thereby eliminating deflection and inaccuracies. Worm gear backlash is thereby reduced to < 0.05°. To achieve the repeatability, you must be moving in the same direction. If you rotate the opposite direction, you need to move beyond your starting point then move toward your target. This removes error associated with the backlash (this is true for any mechanically geared system).

Manually Actuated Units

The Manual DPRP unit comes with a 0.1° Vernier scale as standard.

Stepper Motorized Units

Resolution of stepper motorized DPRP units are approximately 0.3°.

Encoder Options

Motor encoders are available upon request and typically have a resolution of 50nm. It should be noted that these are integrated into the motor as standard, and not on the traveling flange itself. Therefore, while the motor position is accurately measured, one must consider the physical limitation of all stepper motors.

Custom encoder assemblies are available upon request which can be placed on the traveling flange for more accurate positioning, however at this point these are custom solutions only. To request these, please contact MMQR@lesker.com.

Additional Specifications