Design Application Examples Using Linear Shafts

Whether you’re building a new machine or considering making modifications on an existing machine, engineering effort, cost and time are all factors that should be heavily considered before beginning any new project. Why not take a cue from some of your colleagues and see how they’ve applied some designs into their own builds? It’s not always necessary to “reinvent the wheel” and it’s quite possible to minimize the risks as well as cost and time! Though you may need to make adjustments before implementing these into your own designs, you can ensure they’ve already been tested and therefore function reliably.

Application Example 1

This first application design was used for Material Testing. A combination of linear shafts, linear bushings and other components were used to create a Fatigue tester. The upper portion of the work piece holder is connected to two linear bushings that travel along the shaft, allowing for accurate and easy movement of the mechanism. This design uses hardened linear shafts to prevent deformation from occurring between the shaft and bushing while maintaining the precision necessary to properly perform the fatigue test.

Application Example 2

This next example uses linear shafts to create a fixture assembly commonly used in the automotive industry.  The slide unit is driven by a manually operated lead screw assembly that is guided by a set of linear shafts and bushings. The linear shafts are held in place by split shaft supports that clamp onto the end of the shaft. This removes the need for machining at the shaft ends while still providing precise linear motion. In addition, the linear shafts also provide support for the loads applied to the assembly.

Application Example 3

Application Example 3 illustrates a simple way of providing linear motion in a vertical application using linear shafts. In this application, there was no effective way to utilize linear guides since they require continuous support along the entire rail. Utilizing linear shafts allowed for more design freedom so that specialized components could be incorporated (in this particular case the linear shafts are press fit into a metal bracket that serves several functions including shaft support). This helped reduce the total number of parts required to achieve the necessary linear motion and reduce the overall size of the mechanism.

Application Example 4

This application design was used for a vertical product placement machine used in the Electronics Industry. The vertical motion of the inspection platform is achieved with a ball screw assembly. The ball screw assembly is driven by a Servo motor and guided by linear shafts. Since the inspection platform is lightweight, the moment load is minimal and inexpensive, linear shafts can be used instead of linear guides.

Application Example 5

Application Example 5 is an assembly station for smaller electronic components. Air cylinders are used with linear shafts and linear guides to achieve simultaneous horizontal and vertical motion. These linear shafts help displace the applied loads and improve the overall accuracy of the assembly. This is an excellent example of the versatility of linear shafts in applications where it is not possible to use linear guides.

We hope you’ve found these examples helpful and are able to use them as your “designspiration” for your next machine build.

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Part II: Application Examples to Illustrate Ball Screw Selection

Introduction to Ball Screws

The main function of a ball screw system is the efficient transfer of rotational motion to linear motion, sometimes to extremely high degrees of precision, accuracy, and repeatability.  Ball screws are typically differentiated by the manner in which the bearing threads were fabricated, either by a bulk deformation process called rolling, or by a precision grinding process.  For a more detailed discussion about the differences between these two types ball screw systems as general principles useful for selecting between the two types, see the previous blog entry Comparing Rolled and Ground Ball Screws.

However, sometimes far more can be gleaned from specific, concrete examples of common machine applications that utilize rolled or ground ball screws.  These types of illustrations provide the engineer with a few productive, real life illustrations that will serve as general operating principles to aide in the selection process between rolled and ground ball screws for new applications.

Precision X/Y Positioning Tables

Dimensional inspection tables come in a wide variety of designs, shapes, and styles and are widely used in manufacturing, inspection, and other related research applications.  In general, they typically employ a precision ground ball screw system in order to achieve the level of precision required to position the tables for a given application.  The ground ball screws at the heart of both large and small positioning tables are driven by servomotors.  These servomotor/ground ball screw systems are able to attain extremely high levels of precision (1 microinch in certain cases) with excellent repeatability.

Motorized XY Stage

Steppers Used in Lithography Equipment  

Ball screws are also present in step photolithography machines that fabricate cutting edge microscopic integrated circuits which have helped to drive huge advances in raw computing power.  At a high level, photolithography is a microfabrication process that harnesses reactions caused by light exposure to thin film substrates present on silicon wafers.  This process effectively etches geometric patterns onto the wafers, which forms the foundation of the integrated circuit.  In order to accomplish this, a component called a stepper controls the position and location of the light exposure on the wafer and therefore must be able to operate on a microscale at extremely high degrees of precision.

Within this stepper, in a similar fashion as the X/Y positioning tables, the ball screw actuator is paired with a servomotor and a harmonic gear reducer.  Only high precision ground ball screws are capable of achieving the high precision and repeatability that is required to precisely control the light exposure on the wafers to the high levels of repeatability that is required for the process.  These precision ground ball screws operate smoothly, quietly, and efficiently and are the best choice for a machine application that requires a similar level of precision as the lithographic machine application.

Ball Screw Actuator 

Automotive Power Steering

Finally, the most commonly encountered ball screw in everyday life is the typical automobile’s power steering system.  To contrast the previous two examples, the typical automobile power steering system does not require nearly the same degree of precision and positional repeatability that the X/Y table requires.  The rotary motion that the driver applies to the steering wheel rotates the ball screw, translating to the corresponding ball nut which acts as a piston within the pressurized fluid system at the heart of the of the automobile’s power steering.  The ball nut engages the Pitman arm via mating teeth on its underside.  This Pitman arm serves as the primary linkage from the power steering box to the center link that actually moves the wheels.  From this illustration, it’s not too difficult to see how the motion of the ball nut moves the Pitman arm, which controls the motion of the center linkage and the motion of the automobile’s front wheels.

The ball nut engages the Pitman arm via mating teeth on its underside.

For an application like automotive power steering, rolled ball screws are feasible and far more economical due to the fact that the main function of the ball screw is simply to convert rotatory motion into linear motion in the power steering piston.  Because of this, a rolled ball screw nut is more than adequate to achieve the motion translation and lower degree of positioning required for this application.

Conclusion

Hopefully these examples help to provide context for the types of applications that typically use ground and rolled ball screws.  With a firm grasp of the types of applications that may require a precision ground ball screw, the engineer should be far better equipped to select and size the proper ball screw required in order to satisfy a wide range of mechanical demands.

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CNC Part III: Taking the Leap into Automated Machining

The “maker movement” is upon us. Maybe you’re a tinkerer and like new tools, or perhaps you’re an inventor and want to see your ideas come to life, or possibly you’re a maker through and through, and are looking to do more than what your shop is able to handle. You’ve done your research and learned the basics of CNC (computer-numerical control) machining, from the history of the systems to the most common types of machines. You’ve run the numbers, and decided that CNC is the logical next step. You’re ready to take the leap and automate your workshop, right?

Before you rush out and drop some serious coin on a brand-spankin’ new 5-axis CNC milling center, consider the fact that there are now a few options when moving towards automation, whether for DIYers or professional machine shops. Read on for a discussion of the basic advantages and disadvantages of each option.

Building your own DIY center 

Pros

• Fully customizable – You can design your center to fit your size limitations, and make it as minimalist or extravagant as you want.
• Affordable – With the cost of the equipment coming in at pretty much the cost of the components (and your assembly time, of course), a DIY solution can be a good way to get a CNC on a budget. This gives you the option to start with a basic setup and add on over time.
• DIY Project – The satisfaction of completing a project is pretty great. The satisfaction of your project helping you to make other projects? Priceless.

Cons

• Design… and procurement… and assembly… and troubleshooting… – With a DIY project, unless you have a team behind you helping with the various aspects of coming up with a design, sourcing the components, putting it together, testing it, and taking care of maintenance, the brunt of the work and time will be yours to tackle.
• Programming and Controls – Chances are good that if you’re making your own CNC center, you’ll probably be using G-Code, but even if that’s the case, if this is your first foray into machining automation, you’ll end up scouring endless forums trying to find out how to get the code to do just what you want. And once you’ve completed your program, plan on having to further tweak it to get your physical machine to cooperate.
• Time – Between all the research into the components and programming, as well as the delivery and assembly time, plan on having to set aside the jobs for your CNC center for at least a month or more.

Retrofitting an existing machining tool center

Pros

• Easier than DIY – Many CNC manufacturers and integrators offer retrofitting kits and services to allow you to convert existing machines into automated centers, saving you most of the design and assembly time.
• Familiarity – Ok, so you’ll most likely be learning a new controls and/or programming system, but if you’re retrofitting a machine you already own and are familiar with, you won’t have as much work to do to recalibrate or find out how the machine performs for your specific products.
• Added repeatability, precision, and volume – Most machine centers that are retrofitted for CNC automation experience a marked increase in repeatability, precision, and most obviously, part volume output.

Cons

• Limited functionality – Because you’ll be working off an existing machining center, you won’t have the ability to add as many bells and whistles as you might like.
• Ability to retrofit – Let’s face it, not every machine center can easily be converted into an automated CNC center. The cost to convert it may end up being high enough that it just doesn’t make sense to go through with it.
• Mechanical limitations – While automating a manual machining tool typically increases its precision and repeatability, it limited by the tolerances of the mechanical components and assembly. For jobs that require super fine precision, purchasing or making a CNC tooling center may be the only option.

Purchasing a fully automated center (new or used)

Pros

• Ease of selection, installation – With numerous CNC manufacturers and integrators, bringing a new CNC machining center into the fold is as easy as selecting the right line for your needs. Many companies offer to send a technician to install and help get the CNC up and running, in addition to maintenance servicing.
• Replacement parts – As with the services most CNC manufacturers offer, their replacement parts are readily available when something does go wrong.

Cons

• Upfront cost – The initial equipment capital required for a new CNC tooling center can be quite high, especially for an individual hobbyist or small shop. Even used CNC machines can be quite expensive, and oftentimes require additional maintenance due to prior use.
• Space requirements – For many with smaller shops, purchasing a pre-built CNC machining center may mean clearing out some lesser-used machines to make space in your shop. Remember to consider the size of the parts being made, and the flexibility required in the machining.
• Replacement parts – While they may be readily available, they may also be proprietary parts and thus only available through the manufacturer at significant cost. Maintaining control of your design and components offers options to choose more standard parts.

Hopefully going through each of these options’ pros and cons will help you to decide what route to pursue when stepping into CNC machining automation. Remember, there is no right or wrong path, there is only to determine which path is right for you. 

*This list is by no means exhaustive, and there may be even more options for working your way into machining automation. Likewise, there may be additional considerations for your specific scenario that aren’t listed here.

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Performance of Timing Belt Profiles

Performance of Timing Belt Profiles

Determining the exact differences between power transmission (PT) systems can be a daunting task without some guiding clarification about the fundamentals.  Within this quick guide, the fundamentals of the relative performances of the PT drives, flat belts, V-belts, and timing belts– will be examined in order to provide the system designer with a more complete knowledge toolbox that can be applied to the design of even the most complex automation applications.

Chain Drive

One common, inexpensive transmission system that warrants consideration for simple applications is the chain drive system.  This type of drive functions especially well in applications that require a long and potentially adaptable travel length due to the fact that it is relatively easy to simply add more chain links into to the system.  Chain drives, however, are generally more vulnerable to wear failures than belt systems due to the metal-on-metal contact of the mating components; because of this, lubrication of some sort is required to optimize performance and they generally require shorter maintenance intervals when compared with belt drive systems.

Chain system used to transmit power to the rear wheel of a motorcycle

Friction Belts (Flat and V profiles)

In contrast, belt and pulley systems usually do not require any lubrication and as such are able to undergo longer maintenance period intervals.   These drive systems rely primarily on friction to transmit motion, and the two most common belt PT systems are the flat belt and V-belt drives.   In contrast, V-belt profile systems, are literally V shaped and mate in a V grooved pulley or sprocket, and these systems are able to operate systems more efficiently than a flat drive.

While flat belts profile drive systems are usually the slightly more cost effective belt option when compared to V-belt drives, both types are still friction systems and therefore require the high belt tensions in order to properly operate.  These types of friction systems are also prone to slip under higher loads and speeds and should not be used for such applications where such slipping is not desired.  Additionally, since both profiles are friction systems, they are not very mechanically efficient, generate large amounts of heat, and simply aren’t able to transmit as much power as a comparable synchronous belt drive system.

Flat belt drive in the machine shop at the Hagley Musuem

For these reasons, while the V and Flat belt systems are serviceable for a wide variety of applications, their uses are more limited under high performance design requirements such precision demands, a desire for synchronization, and a small, structurally optimized drive system.

Synchronous Belt Profiles

The best profile choice for these performance applications is a synchronous belt drive system.   These belts feature a trapezoidal or circular toothed profile that meshes with a corresponding profile on the pulley or sprocket.   These types of drives maintain system synchronicity far more effectively when compared to flat or V belt profile drives through the use of this toothed profile and mating pulleys or sprockets systems.

Because of the mechanical advantages in the drive system, along with the fact that the belt tooth profiles are comprised of complex geometric shapes that can be difficult to fabricate, timing belts profile systems are also typically the most costly option.

Synchronous drive system

Conclusion

Although the brief survey contained here only scratches the surface of the various types of PT systems performance strengths, it still serves as a solid starting point for even further inquiry into these topics.   The most important point to remember when designing a power transmission system specification is that every application is unique, and there may not be a “one size fits all” belt system that can be employed every time.   Some applications function very well with a basic chain drive, while others may require a highly specialized timing belt tooth profile.

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A Guide to Timing Belt Sizing and Selection

Introduction

Selecting the ideal timing pulley to optimize the performance of power transmission systems can be an intimidating task without some basic knowledge about the various drives available to build transmission systems.  This quick comparative guide will serve as an introduction to the preliminary principles of belt sizing and selection for various drive system types so that the system design engineer can be better equipped to optimize the construction of drive systems.

Roller Chain Drive Systems

The Roller Chain Drive is a common, inexpensive transmission system that can be used in conjunction with a number of various lug and pulley designs.  They are also extremely durable and can be employed in even in the most extreme application environments.  Chain drive systems, however, are noisier and can be more vulnerable to wear failures than equivalent polyurethane or elastomer belt systems.    Because of this, chain and sprocket drives usually require an active lubrication system and generally must undergo more frequent maintenance intervals.

Chain Drive System

Friction Belt Systems (Flat and V profiles) and Synchronous Belt Systems

The two most widely implemented belt profiles are flat belt and V-belt drives.  These belt drives rely on the friction created by the necessitated high design belt tensions in order to transmit motion from the pulley.   The V-belt drive system operates a bit more efficiently and quietly than the flat belt, is able to carry slightly higher loads, and is less prone to slipping than the flat belt drives are.  In comparing the two drive systems, flat belts drives are usually the lower cost drive option when compared to V-belt profile drives because the flat belt profile is an inherently simpler design.

A friction drive system requires a pulley system design that is able to maintain the specified belt tensions that are necessary in order to transfer power reliably without slipping of the belt.  In some cases, these belt tensions may lead to excessive loading on the pulley or sprocket bearings and could result in an overhung load on the pulley shafts.   Because of this, the system designer must perform the due engineering diligence in order to ensure that every component in the drive system is robust enough to handle these loads, which means that the drive system might be comprised of larger structural components when compared to a comparable chain drive.  As alluded to previously, these types of friction systems are also prone to slip under higher loads and speeds and should not be used for such applications where such slipping is not desired.  Additionally, since both types of drives can generate large amounts of heat, additional equipment heat sinks or cooling systems may be required to be designed into the system.  Finally, the biggest downside of frictional belt systems such as flat and V-belt profiles is that they simply aren’t able to transmit as much power as a comparably sized synchronous belt drive system.

V-Belt Drive

For these reasons, while the V and Flat belt drive systems can be serviceable for a wide variety of applications, but their uses are more limited under high performance design requirements such as high speed/load demands where there is also a desire to minimize system noise.  They are also not ideal for applications where design space is limited and a small, streamlined, and structurally optimized drive system is desired.

Finally, although they are typically the most costly drive systems, timing (or synchronous) belt drive systems possess a toothed profile and are ideal choices for high performance applications because they maintain system synchronicity far more effectively, when compared to flat or V-belt drives.  Timing belt drive systems also possess a high power density when compared to flat or V-belt drives, which means that smaller belt and pulley drives can be employed for the transmission of equivalent loads.

However, for specialized applications, they are sometimes the only option because of their power and torque transmission capabilities.

Belt and Pulley System on a 3D Printer

Introduction to Sizing and Selection:

After grasping the more substantive differences between the types of belt drive systems, the system design engineer can move on to determining the size of the drive system needed, based on the desired magnitude of loads that will be transmitted.

The first and probably most important design factor that must be calculated is the system design power (Pd)  To accomplish this, the system design transmission power (Pt), which is the real measure of the power needed to accomplish a particular motion application, is multiplied by a specified overload factor (Ks) which is employed to conservatively account for any dynamic overloads that may be present during the operation of the system.   The derivation of this factor depends on a number of variables such as the type of belt specified, and the full details can be found in the configurable timing pulleys and timing belts catalog.

The second factor to be calculated for the drive system is the required belt width which is a direct function of the design power of the application.  Reference belt width (Wp) which depends on the belt type is multiplied by a factor that takes into consideration the desired design power of the system (Pd), the Reference Transmission Capacity (Ps) and the Engagement Correction Coefficient (Km).  The task is relatively easy to perform, as the standard handbook lays out all the steps in the process, defining the variables, and the equation is represented below.

Conclusion

Although there are other system design factors that need to be calculated in order to complete the drive selection process, these two parameters- system design power and belt width, are a solid starting point to begin building the drive system.   This brief overview of the various types of belt drives that available to the automation system designer should serve as a solid reference point for further inquiry into these subjects.   The fundamental point to remember when designing a power transmission system is that each application is unique, and there is not a “one size fits all” belt drive that can be employed for every possible application.  Some design applications may be ideal for a basic flat belt system, while applications may require a highly specialized timing belt tooth profile.

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