Reducing design time is critical in engineering because the result is lower costs and faster time to market. Design time often includes a number of non-value-added activities such as re-design, over-design, or scope creep that can be minimized by thoroughly understanding all of the application criteria and verifying calculations and analysis via parametric testing of components, modules, and full assemblies with data acquisition equipment, and proving out projected performance results with testing.
Capture as much of the pertinent application information as possible in the beginning to avoid having to go back and repeat portions, if not the entire design process. Be wary and prepared for scope change. Use theoretical calculations and analyses to determine the best initial designs, and then compare them with test measurements of the key performance attributes on actual equipment. Confirm bench test results by performing cycle tests under actual field conditions.
Identifying the Requirements
The first and very critical step of nearly every engineering process is identifying the application requirements. Each product may have a unique set of criteria that will affect its performance. Using a checklist will help to ensure the consideration of parameters that may otherwise be overlooked.
Key application information data in a sample checklist may include:
• Load/speed (dynamic and static)
• Voltage: 12, 24, 36, 48 VDC, 110, 220, VAC
• Direction of load
• Stroke length
• Life/duty cycle
• End-of-stroke protection: Clutch? Limit switches?
• How will actuator be controlled?
• UL, CSA, CE
• Other: Consult your actuator application engineer for additional design considerations.
The selection of the correct ball screw assembly for a specific application can require an iterative process to determine the smallest envelope and most costeffective solution. The design load, linear velocity, and positional accuracy requirements are used to calculate the diameter, lead, and load capacity of the suitable ball screw assembly. Individual ball screw components can then be selected based on life, dimensional constraints, mounting configuration, and environmental conditions.
A good place to start is by defining the direction and magnitude of the load. The system orientation can be very important. With a horizontal orientation, the drive load is equal to the payload weight, times the frictional coefficient. With a vertical orientation, the drive load is equal to the weight. Loads acting on linear bearings and guides can be vertical loads, horizontal loads, or pitch, roll, or yaw moment loads, or any combination thereof. Loads may also vary in their magnitude and direction.
The resultant load vectors at each bearing must be established from the proper combination of the various load vectors to which the linear bearing system is subjected, as life expectancy cannot be estimated based on just the overall system load vectors. The load that each linear bearing is subjected to is called the equivalent load for that given bearing. The system is then sized based on the sizing of the most heavily loaded bearing. For more information on computation methods for an equivalent load, refer to the linear bearing and guide suppliers’ catalogs.
A ball screw assembly, for example, is intended to carry axial loads, translating rotational motion to axial motion. The ability of the ball screw to resist buckling under compressive loads is called its column strength. The screw carries an axial load that is effectively equal in magnitude and opposite in direction to the load imparted to the ball nut — its complementary part — and is related by the design geometries to the driving motor’s torque. In general, the column strength is the limiting design parameter because for longer lengths, it can be much lower than the material’s actual compressive strength. Since the free length-to-diameter ratio is intimately related to column bucking, it follows that for a given diameter, the axial load capacity of a ball screw is dependent upon its free length.
The life of the linear motion system can be predicted based on its operational profile; simply, how many hours per day, days per week, and weeks per year the ball screw will be run. For more complex applications or more refined life prediction, one needs to build a detailed, comprehensive motion profile breaking down the movements to basically straight segments. Each segment of the motion profile would require information as to the speed at the beginning and end of the segment, the time duration of the segment, and the torque during the segment.
Determine the positional accuracy and repeatability that your application requires. For example, inch ball screws are typically produced in two grades – Precision and Precision Plus. Precision grade ball screws would be used in applications requiring relatively coarse movement or those utilizing linear feedback for positional location. Precision Plus grade ball screws would be used when repeatable positioning within microns is critical and no linear feedback device is used. While Precision grade screws have greater cumulative variation over the useful length of the screw, Precision Plus grade screws limit accumulation of lead error, providing more precise positioning over the screw’s entire useful length.
Sizing and Selection
Charts provided by linear motion systems suppliers can be a time-saving shortcut to proper sizing and selection of linear motion systems. We’ll use a three-axis welding gantry application as an example to demonstrate how to select and size ball screws using catalog formulas (Figure 2). The ball screw runs the entire length of the x-axis and is supported on either end by bearing supports. For simplicity, we will define the nut mounting as flanged, the material as alloy steel, the thread direction as right-hand, and the product series as metric. The system orientation in this application is horizontal, with a screw-driven design and the length of the x-axis being 6 meters. It will use fixed ends with a thermally stable flange amount.
A load of 2,668.9 Newtons (600 lbs.) is applied by a carriage riding on profile rails. The travel length is 4.5 m (177.165") and the unsupported length is 5.818 meters (229.055"). The required speed is 0.1 meters per second (3.927 in/sec), and an acceleration of ±2.5 m/s2 (98.4 in/s2) is needed. The duty cycle is 8 hours per day, 5 days per week, and 50 weeks per year with an average of 10 cycles per hour. The life requirement is 20 years for the ball screw and 5 years for components. An additional requirement is that a stepper motor be used due to a preference of the electrical engineering department.
Next, we select the linear bearings for the x-axis. The primary requirements of this application are high load capacity and high stiffness. The application has a relatively long travel length of 5.500 meters (18 feet); however, the availability of 6- meter-length screws eliminates the need for butt joining. Low maintenance is an important requirement of this application. The result was the selection of 500 series ball profile rail linear guides.
With this selection made, the load on the ball screws can be calculated. Based on this loading, the NEFF KGF-D ball nut is selected as the starting point. This ball nut has an integral flange, integral wiper, and a DIN 69051 mounting. The ball screw has an accuracy of ±50 μm/300 mm accuracy.
Next, the life expectancy requirement is checked. Life is typically rated at L10, which represents the time after which 90% of ball screws will still perform. In this application, life expectancy is 1,035,752.6 years. The reason life is so high is that we selected the ball screw based on critical speed rather than life.
Testing the Proposed Design
Once you have selected your design based on the calculations, you need to test to make sure your premises are correct. The testing is designed to validate that what was proposed was actually delivered and, if that was not the case, to guide any corrective actions that may be required. Validation testing should be designed to answer questions such as:
• Does the finished product meet the design specification?
• Is the performance consistent, within experimental limits, with the theoretical calculations, and if not, by how much does it vary and why?
• Does the product provide the required level of reliability?
• What are the potential modes and points of failure for the product?
• How does the current solution compare to alternatives?
For large systems and machines, you may wish to begin with component testing before moving on to bench testing of subassemblies, and then finally to testing of the complete assembly. At each phase of testing, the test results should be reviewed and compared to the theoretical calculations to make sure that the design is on the right track, or consider reasonable opportunities for improvement. Testing is intended to reveal what we might have missed in our calculations and modeling.
Configured Linear Motion Systems
It is also important, all through the process, to consider whether it might make more sense to purchase a configured linear motion system rather than to design and assemble your own. In this case, you would provide the requirements of the application to a linear motion integrator such as the mounting configuration, positioning requirements, environmental conditions, loading conditions, move requirements, and any special considerations. The integrator would then typically utilize a Web-based sizing and selection system to design and configure a custom linear motion system based on your input. The integrator can often provide a quote and CAD file of the proposed design within 24 hours of your request. The cost of such a system will be less than the cost of the individual components, in most cases.
This approach can typically reduce engineering time and assembly cost by 90% or more and can often provide a savings in material cost of 20 to 30%. Most important, by reducing the time spent on designing linear motion systems, your engineers will spend less time working in an area outside of their core competencies, and more time focusing on what they do best — overall system integration.
In summary, take advantage of all useful resources to save design time. Don’t overlook the ability of linear motion vendors to provide configured linear motion assemblies that can help you reduce engineering and assembly costs. Evaluate the alternatives of purchasing components versus modules, versus complete systems in terms of their impact on design and assembly time. Use, to your advantage, all available design tools such as charts, formulas, online selection systems, and 3D models. Finally, engage technical support to leverage their product expertise in standard, modified standard, and specialty solutions. Be sure to confirm that the vendor has design verification/testing/analysis data to back design claims and design positions. This approach can reduce design time to a minimum while ensuring that linear motion systems meet performance and durability requirements.
This article was written by Al Ng, Director of Engineering - Rails, Guides & Components – at Thomson Industries, Inc., Wood Dale, IL. For more information, Click Here .