Tuesday, September 24, 2013

Advanced Scotch Yoke Design Increases Service Life

ROTEX recently replaced some old "orange" scotch yoke actuators with the DRS series. The customer is a major international producer and marketer of food and agricultural products.  The "orange" actuators were experiencing high wear and had to be rotated out and serviced every few months. After the ROTEX DRS actors were installed, at last count, they had 125,000 cycles without any internal corrosion or degradation.

Let's examine the design improvements that allow this superior performance:
  • Pneumatic cylinders are lined with hard chrome plating. Smooth to eliminate wear of piston seals, the surface is very hard, for long life.
  • Springs are lifetime lubricated and epoxy coated.
  • Inside the pneumatic module, dual oversized tie rods are protected from external corrosion. Where the tie rods pass through the piston head, ROTEX has improved the design by providing a slide bearing, where most designs don’t. This bearing is important to help maintain alignment and assure smooth operation.
  • Inside the frame module, the yoke is supported by life time lubricated bearings. Needle roller bearings ensure the most efficient torque transmission. The piston shaft is supported on each side of the frame module by solid bearings that ensure proper alignment and smooth operation.
  • Each piston shaft of the ROTEX DRS is equipped with two Flexijoints. These specially designed joints compensate for movement and changes in alignment to reduce internal stresses and increase output torque. The thrust transmitted through the Flexijoints ensures no side thrust on the cylinder walls and bearings.
  • Piston seal is ROTEX 3X design. Flexibility of the sealing material reduces friction, prevents leakage, and extends actuator life. The 3X shaped seal is wear compensating, and the unique shape of the seal retains the grease for longer periods. The piston is guided by a PTFE thrust bearing, which helps keep the piston centered, and seal wear to a minimum.
  • The tie rod seal is also of the ROTEX 3X design. Old fashioned O-rings are replaced by these flexible, low friction, resilient seals to reduce the failure rate of tie rod seals. Also reduces the wear and tear to the tie rods themselves.
  • ROTEX has also replaced the standard O-ring of the shaft seal. In this position, we provide a proven V-seal design. This self-energized, wear compensating seal guarantees low hysteresis and minimum friction. Like the X seal of the piston and tie rod seals, the inherent shape of the V-seal helps keep the grease where it needs to be.

In 13 months, the ROTEX DRS went 125000 cycles without internal corrosion nor significant wear.

Thursday, September 19, 2013

What is a Rack & Pinion Actuator?

According to Wikipedia, "A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby translating the rotational motion of the pinion into the linear motion of the rack."

Image courtesy of Wikipedia

When applied as a pneumatic valve actuator, opposing racks are attached to pistons in a cylinder. When one side is pressurized, the pinion bearing turns in one direction. When air or spring tension is applied to the opposite side, the pinion bearing turns in the other direction, thus allowing rotational motion in both directions.
ROTEX Rack and pinion actuator
Transparent actuator showing opposing pistons (ROTEX)

rack and pinion bearing
Pinion bearing and rack with piston and seal (ROTEX)
Housing which forms outer pneumatic cylinder walls (ROTEX)
Advantages of pneumatic rack and pinion actuators are compactness and space savings, reliability, durability and life cycle. Disadvantages are gear and bearing wear, piston seal fatigue and stem seal leakage.

Here is a great article on rack and pinion actuators from Valve Magazine.

Wednesday, September 18, 2013

What is a Scotch Yoke Pneumatic Valve Actuator?

According to Wikipedia, "The Scotch yoke is a mechanism for converting the linear motion of a slider into rotational motion or vice-versa. The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part. The shape of the motion of the piston is a pure sine wave over time given a constant rotational speed."
Animation of Scotch Yoke (courtesy of Wikipedia)

In a pneumatic actuator, the scotch yoke mechanism converts linear motion (provided by an air supply and pistons) to rotary motion. A return spring sets the rotation in the opposite direction.

Diagram of Scotch Yoke Internals (courtesy of ROTEX Controls)

Scotch yoke mechanism provides a great torque curve for many valve types and are most often used on larger valves. Additionally, as valve torque requirements increase, a scotch yoke actuator becomes a more economical choice considering production cost.

The disadvantages of scotch yoke actuators are typically the wear and tear on the pin and bearing mechanism. Manufacturers like ROTEX Controls have reduced this issue with innovative design improvements that reduce friction and wear.
ROTEX Controls DRS Scotch Yoke Actuator

Thursday, September 12, 2013

Critical Service, Triple Off-Set Metal Seated Butterfly Valves with ASME Supply Tank, Fusable Links and Manual Hydraulic Over-Ride

16" & 10 " Triple Off-set Valves automated block valves. These are pneumatically operated valves with ASME air tanks for fail close operation on loss of electrical signal. Also included are manual hydraulic over-ride cylinders and spring loaded 3-way diverter valves with "fusible links.

Friday, September 6, 2013

Gas Over Oil Actuators for High Pressure Gas Lines

ROTEX Gas over oil
Gas Over Oil Actuator
Gas Over Oil actuators are used on gas transmission lines where there is sour gas (Hydrogen Sulphide) present. Because of the lack of electricity or other energy sources in remote locations, the pipeline gas is used as the energy source for the actuator, isolating the corrosive gas gas from the wetted parts of the actuator with hydraulic oil.

Tuesday, September 3, 2013

Defining a "Quick Acting" Pneumatic Actuator

ROTEX Fast acting actuator
The desired speed of the pneumatic actuator opening or closing is a consideration of the application itself. For applications where an open valve may feed a fire, or overfill a tank, quick closing is required. Conversely, there are times when lines or tanks need to vented very quickly, or processes must be diverted quickly, and a fast opening actuator is applied.