Fine Adjustment Screws
Introduction - Fine Adjustment Tutorial
The term 'high-end adjuster screw' generally applies to 127-254 TPI (Thread Per Inch) matched ultra-fine thread screw and nut/bushing integration used to provide extremely precise axial linear positioning as a result of turning the screw a specified number of degrees. High-end adjuster screws are described by the screw diameter size designation as well as the exact linear displacement per single revolution of the set. Such high-end adjuster screw sets are used in a wide variety of precision instrumentation, fine mechanism adjustments, and metering devices.
The high-end adjuster screw pitch#, thread class#, and thread quality determines the resolution of the integration. Typical high-end adjuster screw pitch specifications range from 0.1mm to 0.2 mm per revolution, or 127 to 254 TPI.
Fine adjuster screws generally apply to 40-100 TPI screws and nut/bushings where interchangeability# and cost of the assembly are key parameters. The thread class for these fine adjuster screws is typically 3A/3B, the tightest thread class. Note that Base Lab Tools offers adjusters above class 3A/3B with better performance results and reduced need for “sticky” grease to accommodate for loose tolerances. These devices have less backlash and longer life expectancy than standard 3A/3B adjusters.
TPI (Threads per Inch)
Epoxy Free Design
Adjuster screws offered through Base Lab Tools are designed for performance and cost. That means all aspects of the design have been reviewed to ensure that performance is never compromised. An example of this effort is a true epoxy-free design to get the very best centration of the ball tip. By removing the need for epoxy, the operating temperature range for the adjuster is increased as well as the long term stability of the product. It should be noted that some competitors are offering different epoxies in the adjusters to enable it to work in different applications e.g. vacuum or high temperature.
The nut or bushing also holds great importance to the performance of high-end adjuster screws. Not only are the length and class of the nut/busing important, but also the material itself. In low cost adjusters where grease can be used, and wear relative to load is small, brass is typically the most economical material to use. In high-end adjusters where tight tolerances are required as well as the capability to withstand wear and higher loads, phosphor bronze is the preferred material. Phosphor bronze is considered a self-lubricating material, making it ideal for grease free applications such as vacuum environments. Still, the use of grease will extend the lifetime of the nut/bushing and is recommended unless a grease free version is required.
In general terms, a longer nut or bushing will have better performance than a shorter one if pitch and yaw of the adjuster screw is of importance.
In high-end adjuster screws, Base Lab Tools uses phosphor bronze nut/bushings to allow for high loads and grease free configurations upon request.
Movements of the adjustment screw are typically made using a thumb screw or an Allen key. The adjustment interface can often be made compatible with fully automated solutions such as gear boxes, capstan drives, etc.
Adjuster screws are available in single- and dual-contact. Single-contact screws have a looser fit between the screw and nut components so that only one side of each course of threads on the screw contacts one side of the corresponding thread of the nut/bushing. When the engagement is rotated in the opposite direction, the other side of the respective thread is in contact. These are meant to turn quite easily with minimum turning torque. The dual-contact type applies contact to both sides of each thread, to the point where there is zero axial lash or linear motion possible with mere axial tugging. The only axial motion possible is by rotating the screw engagement.
High-end adjuster screws are used to finely control micro valves used to meter trace amounts of gas or liquid in chemical analysis, medicine, industry, and aerospace. Such valves are often used to control the flow in gas chromatographs, mass spectrometers, gas mixers, and other gas analysis equipment.
Press Fit Method
The assembly process begins by producing the appropriate sized mating hole for the threaded nut/bushing. We recommend the use of a reamer to achieve a circular hole with the proper diameter. Given the close fit between the threaded nut or bushing and the mating adjuster screw, we cannot stress enough the importance of making a circular hole with the proper hole diameter. The hole should be 0 to 0.0003” smaller than the nut/bushing. For our own production needs, we combine the press fit method with a permanent anaerobic adhesive e.g. Loctite 603, 609 or equivalent that provides excellent long-term performance. It is essential that both surfaces be completely free of grease for proper curing of the adhesive.
The nut/bushing should be put in place using a press tool or similar method to ensure that the nut/bushing is put into the hole straight. If the nut/bushing is deformed, the adjuster screw may get stuck in the nut/bushing during use.
As shown in Figure 1, it is important to mount the nut/bushing such that any axial force is taken up by the mounting plate. Axial force should not be carried by the adhesive since creep may occur.
The benefit of the press fit over a loose fit is the unbeaten performance of the nut/bushing mounting surface joint.
Loose Fitting (Epoxy Method/Adhesive Method)
The assembly process begins by producing the appropriate sized mating hole for the threaded nut/bushing. For our own production needs, we use a permanent anaerobic retainer that provides excellent long-term performance.
We recommend using Loctite Anaerobic Retainer product numbers 609, 648, 680 or equivalent. In order to ensure maximum strength between the bonded parts, we strongly urge the user to carefully follow the instructions provided by Loctite, which specify the amount of clearance between the threaded nut/bushing and the mounting plate. It is essential that both surfaces be completely free of grease for proper curing of the adhesive. The most common source of failure is not allowing for the proper clearance between the parts being bonded since this clearance provides for the proper volume of adhesive.
It is important to mount the nut/bushing such that any axial force is taken up by the mounting plate. Axial force should not be carried by the adhesive since creep may occur.
When assembling, apply a small amount of the Loctite adhesive to both threaded nut/bushings and the mounting plate and ensure a uniform distribution of the adhesive around the perimeter of the parts before assembly. After insertion, remove any excess adhesive from around the part.