A Primer On Worm Gear Operators

Over the past 30 years, valve automation has dominated the flow control industry. But even though power actuation captivates the attention, imagination and the lion’s share of growth in the market, manual valve actuation also continues to expand, receiving its own innovations. In this article, we provide an introduction to the basic principles behind these workhorse operators and discuss current trends in the market. We also consider the benefits and associated costs that come with manual worm gear designs. Please note that for ease of reference, a glossary of terms has been added; the definitions provided are industry specific and only intended to cover the depth and scope of this article (click here).

THE RANGE

Manual worm gear operators can be found in nearly every valve application throughout the world. Manual operators have no power requirements, no hydraulic or pneumatic pressure unit to maintain and can be used in the most remote locations. From submarine duty to mining, water works to oil and gas pipelines, if torque is required, manual worm gear operators are there.

To begin, it may help to view the world through the eyes of the application engineer. We push up our stylish horn rim glasses, open our 20-tab spreadsheet product selector and ask: “Torque or thrust?” The first consideration in selecting an actuator is the type of force required. Torque, that rotational or twisting force necessary to position ball valves, plug valves, butterfly valves, etc., will be the focus of this article.

Let’s look at the fundamental challenge our application engineer faces, which is: “How do we provide a means to safely and effectively position the valve?”

We have used gears for thousands of years to harness energy from wind, water and beasts. Think of gear mechanisms as a series of interacting levers and screws. In our application, gears are used to amplify torque. In short, we use gears to convert force to work.

THE MECHANISM

Following conventional American Gear Manufacturers Association (AGMA) (www.agma.org) gear design standards, if a single-start worm is the drive and an 80-tooth worm gear is the driven, the ratio would be expressed as 1:80 (the formula is available in the glossary). That’s a lot of ratio in a small package. But these numbers only tell us that it takes 80 worm revolutions (drive) to complete one revolution of the worm gear (driven). The ratio tells us about the mechanism’s effect on speed but little about torque. To understand the effect on torque, we need to know the mechanical advantage (MA). In a perfect machine, a 1:80 ratio would net an MA of 1:80 (expressed as 80), meaning that for 1 unit of force applied, 80 units of force are generated.

Does this mean free torque? Unfortunately, no. The energy of the universe is constant and as with everything, there is an associated cost.

THE COSTS

What are the costs associated with amplifying force through our manual actuator? The first cost is hard cash—the worm gear actuator will cost more than a lever or handwheel.

Second, machines are not perfect, which is demonstrated in efficiency losses through heat and wear. As it turns out, standard single-start metal worm gear sets are less than 35% efficient by design. Also, worm gear sets with a worm thread angle and design, which net an efficiency of approximately 35% and greater, are not typically self-locking, and without self-locking characteristics are not suitable for a number of applications. Based on a website sample of the top manufacturers, 32% is the approximate average efficiency for manual worm gear operators. This simple formula explains what happens to mechanical advantage in a 32% efficient gear train:

80 x .32 = 25.6 MA

The MA would be approximately 25, not 80. That is nearly 55 points of mechanical advantage lost to inefficiency.

Third, and not captured in manufacturer’s data sheets, is the cost in the exchange of force for distance. We increase force by simply making more trips, or more specifically, more turns on the handwheel to cycle the valve.

SELECTION CONSIDERATIONS

With few exceptions, base worm gear operators are fairly simple creatures. Besides the specific torque requirements and duty-cycle rating, variation in selection comes primarily from valve adaptation, service environment and options.

Adaptation: Adapting the operators to the specific stem and top mounting flange or bonnet of the valve presents the greatest opportunity for error. Valve topworks are as standard as tightly tolerenced snowflakes, with each valve manufacturer and engineer bringing their own unique stem and bonnet configuration to the market.

Environment: From the North Slope of Alaska, to the sands of the Arabian Desert, to harsh chemical exposure, service environment plays a major role in determining the type of materials, coating, hardware, grease and seals, selected for the application. Understanding the service environment and adapting to it is critical to the service life of the actuator.

Options: Remember the 20 tabs on the application engineer’s selector? She needs these to make sense of optional features available on manual operators. Hand-wheel orientation, direction to close, lock open, lock closed, torque input limiting, mechanical stops, tamper-proof stops, remote position indication, chain wheel operation, input shaft drive adaption, buried service… the options are endless and not a subject for this article. A look at a basic manual operator, however, will provide an introduction to terminology and improve understanding when communicating specific application requirements.

MATERIALS AND DESIGN

In general, bronze is a better gear material than iron; however, selecting or disqualifying gear operators strictly on the basis of material is a fool’s errand. Gear operators are sliding, rolling, dynamic, friction funhouses. Gear geometry, clearance, tooth engagement, mating materials, gear alignment, bearing design and contact pressure are just a few of the variables that determine whether a particular material is suitable. The other considerations are environment, application and the manufacturing process. Poorly designed gear operators and variation in manufacturing can turn the best pedigreed bronze and hardened steel to scrap. A robust design, built by controlled manufacturing processes, supplied with qualified materials (be they cast iron or forged bronze), are more reliable at determining performance. Working closely with a reputable manufacturer to ensure the right gear operator is selected for a well-defined application offers the best insurance against material failure.

INNOVATION

As valve manufacturers make larger and more sophisticated valves with higher torques for remote locations that have limited infrastructure and technical support, manual gear operators remain a viable, age-old solution with a few 21st century twists. Understanding the tradeoffs and costs associated with manual operators can help end-users and valve manufacturers select the right technology for the application.

Keith Pierrotti is plant manager for the Distributed Valves Division, Cameron (www.c-a-m.com), Limerick, ME. Reach him at kpierrotti@c-a-m.com.