IMI STI MC Linear-to-Rotary Motion Converter

IMI STI MC motion converter: what it is

The IMI STI MC is a mechanical motion converter. It takes the up-and-down movement of a linear valve stem or actuator rod and turns it into a 90° rotary output, so that a rotary positioner, position transmitter or limit-switch box can be fitted to a linear control valve. Many positioners are designed with a rotary shaft input; the MC provides that input without a long exposed lever.

MC is the model name given by the manufacturer. The brochure lists two housing materials and a coupling kit that can be adjusted or exchanged to suit different accessories, rather than a long ordering code. Seven Star LLC, an independent supplier in Muscat, quotes the MC for Oman and the GCC against your actuator stroke and accessory details.

MC technical specifications

How the MC converts motion

On a conventional installation a positioner on a linear valve is driven by a lever and a pin that follow the stem. The lever swings through an arc, the pin slides in a slot, and the geometry produces a feedback signal that is not perfectly proportional to stem position. On short strokes the effect is worse, because a small linear travel has to be turned into a usable rotation of the positioner shaft.

The MC replaces that arrangement with a lever mechanism enclosed in a small box. The stem or rod movement is picked up by the mechanism and delivered as a 90° rotation at the output shaft, with a design aimed at keeping linearity error low. A backlash recovery system takes up clearance so the feedback does not dither on direction reversals, and the self-cleaning action helps the mechanism stay free in dusty or icy conditions.

Where the MC fits in the system

The MC is an accessory, installed between the actuator and the feedback or control device. Typical partners are rotary-shaft analog and digital positioners, electronic position transmitters such as the IMI STI 2030, and limit-switch boxes. The coupling kit is designed so that the mounted accessory receives no radial force, which protects the accessory bearing and gives a steadier signal over time.

It is used with single and double acting linear actuators, for example the manufacturer SC/V linear actuator and SC damper actuator that appear in its related-products list. Whether a given positioner can be coupled depends on the coupling kit, so state the positioner model when you enquire.

Selection and ordering checklist for MC

• Actuator stroke: the standard cam suits strokes above 70 mm, and the linearity figure applies from 90 up to 400 mm.
• Housing material: anodised aluminium (1.2 kg) or stainless steel 316 (2.7 kg), depending on corrosion exposure.
• Positioner, transmitter or limit switch to be coupled, with its shaft and mounting dimensions.
• Ambient temperature, with -40 to 120 °C as the listed range.
• Single or double acting actuator and the direction of motion on air failure.
• Mounting interface on the actuator yoke or bracket.

Installation and maintenance of the MC

Mount the MC on the actuator so that its pickup follows the stem along the full stroke. Set the cam to the actual stroke in the field; the brochure notes that the device can be adapted to any stroke on site, which helps when the final stroke is only confirmed at commissioning. Fit the coupling kit to the positioner shaft, check that nothing binds at either end of the travel, then calibrate the positioner in the normal way.

Because the mechanism is enclosed, there is little to service. The manufacturer describes it as maintenance free, with a robust construction not affected by dust, ice, temperature or other contaminants. A visual check at turnaround for loose mounting bolts and a check of the coupling are sufficient in most plants.

Typical applications in Oman and the GCC

Linear control valves in refineries, gas plants, desalination plants and power stations often run in heat and blowing sand. An open lever-and-pin linkage collects dust and can be knocked by maintenance staff; an enclosed converter avoids both. The MC is also used where the stroke is short, such as small globe valves, where a standard lever would give poor resolution to the positioner. The upper temperature of 120 °C gives margin on valves close to hot lines.

Benefits of the MC compared with a lever arrangement

Information to send with your MC enquiry

• Actuator make, model and stroke length.
• Positioner, transmitter or switch box model to be mounted.
• Housing material preference.
• Ambient temperature and site conditions.
• Quantity and delivery location.

Documents and quotation

A manufacturer brochure for the MC is attached on this page, including the technical specifications and benefit list. An instruction manual is published by the manufacturer; ask us if you want it before installation.

Send the part number MC with your actuator and positioner details and Seven Star LLC will prepare a quotation for Oman and the GCC.