Why Detailed Mechanical Specifications Are Imperative to Test System Success

Abstract

If your product is often used as a component in a larger custom mechanical system, you may or may not have spent much time considering how your solution fits within the larger picture.

In order to increase the likelihood of system success for your end user – and a greater perceived ease-of-use and satisfaction with your products – you should consider including a detailed mechanical requirements specification.

But where do you start, and what are the best practices for creating this
documentation?

Complete Specifications Are Critical to System Success

Purchasing custom equipment is very different from purchasing off-the-shelf items, due to the fact that it’s much more difficult to qualify suppliers and to compare offerings from multiple suppliers. Since there is no uniform, widely accepted standard for the design of custom test equipment, your specifications are critical in setting the standards and expectations for your system.

Including accurate and detailed mechanical requirement specifications helps ensure that a custom system works as required without costly corrections later in the process. Incomplete specifications can often result in sourcing lower-cost solutions that unfortunately will not actually perform as required. In fact, many of the problems associated with mechanical test systems can be traced back to improper or poor
equipment specification.

For example, when equipment is purchased without a detailed mechanical specification, a customer must rely entirely on the engineering team or the integrator to fabricate a system according to its own standards or procedures. These standards may be adequate for operating conditions that a supplier considers “normal,” but may not be adequate for requirements that fall outside these conditions. Since there is no widely accepted standard for “normal,” standards need to be defined as they apply to your application. Failure to do so may result in the supplied equipment not performing as needed in your application, making them unsuitable for the intended use.

While the supplier you select should be an expert at designing custom equipment and be able to demonstrate that expertise during the sales process, you should not expect them to be an expert in regards to your part or test requirements. You will need to fill in the blanks about the unique characteristics of your part and the test required, and a well-written mechanical requirements specification is your best opportunity to communicate critical part or process information.

In addition, a properly prepared mechanical requirements specification is integral to your success because it helps define the performance requirements of the test system’s mechanical elements, materials of construction, fabrication methods and procedures, tolerances, test and inspection requirements, and so on. If there is a problem with the performance of the mechanical test system, the mechanical requirements specification will serve as the primary reference document to solve the issue.

Quantify Wherever Possible

Ideally, a mechanical requirements specification should:

• Be as short and concise as possible
• Focus on the required end results
• Include detailed prints of any parts that interface with the mechanical
system
• Delineate the methods for verifying system performance

Keep your mechanical specifications as concise as possible while still including all key information. When mechanical specifications become too verbose, you run the risk of integrators missing key requirements in a lengthy document. One tip is to cross-reference other standards and codes wherever possible to avoid repeating sections within the document.


Enough detail should be given in the specifications to clearly communicate what is required from the integrator, while at the same time not unnecessarily restricting the pool of available vendors. Avoid using subjective statements such as “high-quality” and “easy-to-use.” Mechanical system requirements need to be objective and factual. Subjective statements are open for interpretation and difficult to evaluate. A useful rule associated with specification is the mantra:

“Quantify wherever possible.”

In developing a mechanical requirements specification, set bounds on a wide range of relevant parameters for the product or process concerned. Reliable, safe, mechanical test systems generally provide the following functions:

• Details about the Unit Under Test (UUT) including toleranced positions and required orientations

• Repeatability and accuracy requirements of mechanical motions • Information on the durability of motion elements for anticipated life cycle of the test system

• Guarding preferences used to protect system operators, test system components, and facility environments

• Specs on the maintainability of system • Information on test system integration/interfacing with existing facility elements

4 Key Questions for Mechanical System Development

Each of the functions listed above becomes a focus point during the development of a mechanical requirements specification. The particular needs of a new test system drive mechanical tolerancing, selection of motion elements, design of the support structure and guarding elements, and maintenance accessibility. Important questions to ask during the development of mechanical systems include:

1. What are the required performance specifications necessary to properly achieve the end result of the product/process test requirements? Applications with mechanical components may have requirements in terms of speed, torque, load, pressure, flow, and part-to-part test cycle rate. Depending on the performance specifications, the mechanical system may include high-speed or high-torque motors, hydraulic or pneumatic cylinders, gearboxes, linear or rotary actuators, and so on. Specifics about the minimum and maximum ranges for performance specifications need to be outlined in the requirements, along with applicable tolerance bands (e.g., speed-torque curves for dynamometer test systems, and hydraulic manifold port specifications for automated, end-of-line OEM transmission test systems). It is important not to over-specify these parameters because the higher the system performance, the higher the cost. For example, rotational speeds over 5000 RPM (especially over 10,000 RPM) may require custom motors and expensive, balanced drivetrain components.

2. How accurately do the process variables need to be controlled and measured? The required measurement accuracy of these components will also affect the mechanical hardware selection. For example, tight tolerance on displacement measurements will affect the selection of supporting linear slide and bearing assemblies in order to minimize errors introduced by the clearances in the motion stage supports. If this performance requirement is coupled with high loads, any supporting structure will need to be designed to minimize measurement errors due to mechanical element deflections. Be aware that high accuracy and tight tolerances will drive the cost and uniqueness of a mechanical test system, so demand the best only where absolutely necessary. Remember that when specifying tolerances or accuracies that each number after the decimal will add cost (i.e. tolerances of 0.01” will be less expensive than 0.001”). Do not build safety factors into your requirements; instead state that the mechanical designer should account for these factors in his or her design.

3. Does the fixutring or tooling need to be frequently changed? If so, clearly define these requirements in the mechanical specifications. Changeover times, tooling requirements, assist mechanisms (such as air tools and overhead lifts), pallet handling mechanisms, and weight restrictions should be included to help the integrator understand the environment in which the test system will be operating. Clearly identify whether or not the system operator will be responsible for system reconfigurations, or if the test system will include semi- or fully automated changeovers. Again, cost and machine complexity will increase as the level of machine automation increases.

4. Does the system require vibration isolation or vibration monitoring for predictive maintenance or alarming?

Mechanical test systems that perform repetitive, cyclic motions will degrade over time as the elements responsible for the motions experience abrasion along moving surfaces (e.g., linear bearings, roller bearings, and gear mesh). Proper maintenance of lubrication systems is vital to longterm operation of these types of test systems.

Clearly specifying expectations for the system lifecycle will require potential vendors to make appropriate decisions in selecting hardware that meet the mechanical requirements of their test systems. Validation of life cycle requirements can be demonstrated during proposal reviews and project design reviews by utilizing estimations and calculations provided by the integrator. Additional requirements may include vibration monitoring systems to help provide a level of predictive monitoring within the system. As machine elements degrade, the vibration signatures will change within rotating elements. A monitoring system can often be used to track machine vibrations and trip a system alarm when vibration levels become excessive.

In critical applications, normal vibration operating limits should be specified and monitoring systems should be included in the test system requirements. This will clearly communicate to potential vendors the requirements for appropriate vibration monitoring sensors and controls, as well as the requirements for robust motor, gearbox, or rotating elements in the mechanical design of the test system.

Mechanical Design and Validation Tools

Mechanical design assembly tools are vital during the design phase to ensure that the mechanical design will stand up to the requirements of the final deployed system. Depending on the data required in this phase, different design and validation tools are available, such as:

• Computer-aided design (CAD) software

o Autodesk Inventor 3D

o CATIA

o PTC Creo (formerly known as Pro/ENGINEER)

o SolidWorks

• Modal, static, rotor dynamics, thermal analysis

o ANSYS Structural

o COSMOS

• Laser alignment

• Portable coordinate measuring machines (CMMs)

• Vibration analysis

Since these tools can be vital to the completion of a successful mechanical design in accordance with published mechanical system specifications, it may be important to mention in your specifications that 3D CAD models and finite element analysis (FEA) results will be reviewed during the design phase of the project. These types of notes are often included in the requirements section that outline project execution. This analysis can often be a cost driver for the supplier, so make sure that all the suppliers understand your requirements in this area and include them in their quote to ensure quotes can be compared on an even playing field. Regardless of what level of analysis you require, it is considered a best practice to have a design review of the entire system prior to releasing the design for build.

Even Tables Need Technical Specifications

Even simple items require technical specifications. The integrator may need to know requirements, such as:

• Material of the frame

• Material of the table top

• Adjustable or fixed height

• Surface area

• Mobility (if it needs casters or will be in a fixed position)

• Tolerance of the surface flatness

• Weight it will be required to support

If the mechanical specification provides insufficient detail, the vendor may offer the cheapest available solution that meets the provided requirements, affecting equipment durability, repeatability, or measurement accuracy. Problems like this are often discovered late in the system fabrication process – during final assembly and system validation – making them considerably more costly both in terms of time and expense, compared to discovering them earlier in the design phase.

Conclusion

Creating a test system with mechanical components and housing introduces a
whole new layer of planning in the design process. But the effort is worth it, as it
helps you avoid making costly mistakes in the execution phase. By documenting
a list of requirements with quantifiable and detailed specifications and working
with an engineering team experienced with constructing mechanical systems,
you can ensure that the products included in the final test system will perform as
you need them, when you need them, with no surprises later.

Need Help Building a Mechanical Test System?
Wineman Technology has years of experience designing, constructing, and
integrating mechanical systems for data acquisition, control, and test. For help
building a comprehensive requirements document for your application, contact
us today.