IST Designs Leverage Flexure Bearings for High-Precision Instrumentation

IST Precision develops a range of industrial automation and high-end precision instrumentation devices. This article highlights the use of flexure rotational pivots in some of our more sensitive precision instruments, focusing on applications like surface profilometer hinges and compact steering laser gimbals.

We’d like to showcase C-Flex, a vendor we rely on for flexure bearings. These bearings enable exceptionally precise and frictionless motion in well-designed mechanical devices.

In precision instrumentation, achieving smooth motion at the nanometer or sub-arc second level is crucial. Understanding the need for frictionless motion devices when aiming for nanometer type motions is critical. The bottom line is friction in rotary joints can cause nonlinear motion and mechanical hysteresis at a nanometer level. This leads to a poor performing device due to these unwarranted errors that can occur. Several precision rotary-motion bearing options exist, including air bearings, jeweled bearings, and ceramic bearings. For applications demanding near-zero friction and smooth, highly linear motion, air bearings are a great choice, but they can be expensive to design and operate, requiring dry air. This adds cost and takes up valuable space.

In situations with limited space (i.e. satellite industry) and only a few degrees of limited angular range the designer can look to an alternative pivot called a flexure bearing. They offer zero friction, compactness, and no air requirement. However, their rotational stiffness is fairly low and the angular range is typically under +/- 10 degrees.

IST has extensive experience using flexure bearings for many years, alongside designing linear custom flexures for over two decades. We often utilize 17-4 SS with an H900 heat treatment to achieve exceptionally high yield stresses (greater than 1000 MPa). While C-Flex’s material specifications are unknown, using materials with high yield stress is paramount to prevent pivot deformation in the thin metal webs.

These flexure pivots are reasonably priced, typically costing between $50-$100 per unit in low quantities. Pivot costs depend on size, with some of our mechanism designs typically using some of the smallest flexure bearings offered (1/8 mm or 3.125 mm barrel size).

The C-Flex website offers valuable resources for the instrumentation designer ranging from bearing loads to clamping methods. We particularly appreciate their laser engraving of part numbers on the outer diameter of the pivots. This is not offered by other compitorers. This simple laser marking helps prevent mix-ups in the assembly process that might use multiple pivots with varying torsional stiffnesses.

The pivots come in two main body types: fixed-load and fixed-load-fixed. The fixed-load is simple one end of the bearing is clamped to the device’s fixed frame while the opposite end is able to flex and rotate. The fixed-load-fixed type bearing is where opposite ends need to be clamped to the fixed frame and the mid section is able to freely flex and rotate. The choice depends entirely on the instrumentation designers application. We have used both types of bearings in designs over the years.

Finally, we’d like to highlight three common clamping methods for these pivots:

• Squeeze Clamp: This traditional method mechanically clamps around the pivot. While effective for some designs, it requires more design space and often screws, which might not be feasible in all cases.

• Set Screws: These provide an alternative but carry the risk of pivot slippage. If using screws, opt for soft-tipped ones.

• Bonding: This offers the most compact approach, ideal for designs requiring minimal parts and extreme compactness. However, it also carries the highest risk.

One interesting design we can share is shown below and presented as a CAD model. The small shaft is 1.2 mm in diameter with a custom diamond tip bonded into the end of the rod and protrudes out the side through a microscale hole. As shown the shaft is designed onto a small rotating mechanism which employs a flexure bearing pivot inside the green body. We are able to detect motions of the shaft relative to the green body down to single digit nanometers using custom displacement sensors called a differential capacitance sensor. While the sensor itself is a key part of this design the flexure bearing enables our rod to very accurately move without any friction. Using precision motion stages (not shown) enable our device to scan along a surface and by monitoring the sensors we are able to acquisition the sensor are very fast sampling rates exceeding 10,000 samples per second. Ultimately we have developed a very precise simple mechanism capable of tracking nanometer finishes.

We hope this article provided valuable insights into flexure rotational pivots and their applications in precision instrumentation. Feel free to reach out if you have any questions!