This module identifies the design considerations that should be considered when designing a mechanism. This module is intended to apply to all aerospace mechanisms, including flight control, door mechanisms, throttle mechanisms, landing gear, etc. For more details on mechanical flight control system design see Flight Control System – Design.
When designing a mechanism from scratch without any prior system or experience to guide the design, the design task can be initially overwhelming. Additionally, in literature, there is not much information on how to approach mechanism design. A basic approach to mechanism design is shown in Figure 1. Following this flowchart will help keep the design process on the right track. The amount of time spent in each phase of the process depends on the expected complexity of the mechanism, similarity to other designs and the amount of information that is initially available.
Figure 1 Basic Mechanism Design Process
Following Figure 1, the design steps are described as follows.
Step 1 – The purpose of Confrontation/Requirements Definition is to take a big picture view of what you are trying to accomplish with the mechanism and define a basic approach (or the general nature of the mechanism) to accomplish the objective. At the same time, the design requirements of the mechanism should be determined. Requirements will include the kinematic relationships for position, velocity and force/torque transfer. Requirements should also include weight, limit load, duty cycle, backlash, freeplay, stiffness, friction, accessibility, manufacturability, environmental and so on. Requirements will likely evolve as the design matures.
Step 2 – Information Gathering can be done in parallel with Confrontation/Requirements Definition. The type of information that would be “gathered” would be information on similar designs to include performance characteristics, reliability, weight, etc. Vendor catalogs for such things as bearings, cables, gears, etc can also be used to obtain information. The intent of the obtain background information activity is to obtain sufficient information to make good decisions and accurate tradeoffs to support the development of the viable design concepts.
Step 3 – At the Design Concepts phase, basic design concepts are defined and tradeoffs done. Here you would decide whether to use linkages, gears, cams, cables, etc. in the mechanism and where they would be located in the mechanism. Also, if lost motion, dwell or sequencing is required during some portion of the mechanism motion, this aspect should be identified during this phase. Experience and intuition with mechanisms play a large role when generating concepts. General CAD layouts can be done at this stage to support development of design concept(s). Conceptual evaluation tools such as the Pugh Concept Selection methodology may be helpful for evaluating different concepts. As a general comment, better designs are obtained when multiple concepts are developed and a concept selection process is used. At the end of Design Concepts phase, basic layouts, either in CAD or via sketches should be prepared for the selected design concept that is expected to meet the overall requirements.
Step 4 – In the Synthesis phase, the performance (kinematic) aspects of mechanism are determined. Recall that kinematic techniques are used to design mechanisms. In this phase, the kinematic characteristics of each portion of the linkage and the overall linkage will be determined. Hence, the gearing ratios will be computed for the range of motion of the mechanism. Cable stretch characteristics, cam shapes, gear sizing, etc. are aspects that need to be defined and included in the Synthesis phase. Mechanisms software to evaluate and understand mechanism characteristics would be very helpful here. Certainly any type of mathematical model of the mechanism will be helpful. In some cases, a mathematical model of the mechanism is required. During synthesis, the mechanism design should be evaluated against the design requirements from Step 1.
Step 5 – Optimization involves detailed analysis of all aspects of the mechanism and refining the design to meet all design requirements. During this phase, items like a undesirable characteristic in the mechanisms during a portion of movement (from a nonlinearity for example) would be examined and any improvements implemented. All parts should be evaluated for loads using stress or finite element analysis. Freeplay, stiffness, weight, friction, reliability are other characteristics that should be evaluated. Clearances and accessibility when installed on the vehicle should also be assessed.
Step 6 – Finalization involves completing the drawings and any analysis/reports required to document the final mechanism design. Also, during finalization necessary performance, load and environmental qualification testing should be performed.
When designing a mechanism for a given application the following parameters should be considered during the design process. Depending on the nature and function of the mechanism, the importance of each item on the list below will vary from one mechanism to another.
Velocity (Gearing) Ratio – The primary performance criteria for mechanism is the gearing ratio. Gearing ratio is the same as the velocity ratio. Since mechanism behavior is nonlinear in general, the gearing ratio varies throughout the movement of the mechanism. Therefore, gearing ratio should be evaluated at all possible positions of the mechanism. Gearing ratio can also be affected by structural compliance (stiffness) in the mechanism and thus structural compliance should be considered when determining gearing ratio under load conditions. If cables are used, cable stretch should be accounted for when computing gearing ratios.
Force/Torque Transfer – Force and/or torque transfer through the mechanism is related to gearing ratio. Usually the maximum input force is specified and the corresponding maximum output force is specified. This actually determines the minimum required gearing ratio (over the full range of travel) that is used in design of the mechanism.
Load, Limit (or Max Operating) – Maximum operating or limit load will size components through a static strength analysis. Limit load may be same as maximum operating load or may be slightly higher than maximum operating load, depending on how the terms are defined in the specification. Ultimate load is 1.5 times limit load. At limit load, the mechanism must operate properly and meet all performance requirements. At ultimate load, the mechanism must remain intact but some permanent deformation is acceptable. The mechanisms does not need to operate after application of ultimate load.
Endurance and Fatigue Loads – Endurance and fatigue loads represent the normal operating loads that a mechanisms would be expected to see over its operating life. Endurance loads are normally much less than limit load. Endurance and fatigue loads are usually the same spectrum. Endurance is associated with the mechanism operating and fatigue loads are associated with the mechanism held statically. Like static loads, fatigue loads are used to size mechanism components through fatigue analysis. Mechanisms with high load and low cycles will likely be sized by the static load while mechanisms with requirements for high operational cycles will likely be sized by endurance loads. Endurance loads are normally associated with cycles (i.e., so many cycles at one load, so many cycles at another load and so on). Endurance tests are normally done with the mechanism operating and fatigue loads are applied with the mechanism static (not operating). Endurance testing is done to prove out wear characteristics and the ability of the mechanism to not develop excessive freeplay in service. Fatigue requirements will specify the number of lives that a mechanism must be tested. For example, if 1 life of endurance and 4 lives of fatigue testing are required a common practice is to do 1 life of endurance (mechanism operating with endurance load spectrum applied) and 3 lives with the endurance/fatigue load applied with the mechanism not operating (held statically).
Input/Output Travel Limits – Mechanisms are designed to operate over a required operating range. Stops must be installed at either the output linkage or the input linkage or both to ensure the mechanism cannot be operated outside of the design travel limits. The stops should be sized for limit load and ultimate load capability.
Friction – Friction occurs whenever there is a rotating component (such as a bushing or bearing or cable rubbing on a fairlead). Friction should be minimized as friction leads to inefficiency. However, some friction is often desirable to hold a mechanism in place and to provide some force feel when operating a mechanism under no load conditions. Selection of low friction bearings, eliminating any possible rubbing or chaffing and minimizing the number of components in the mechanisms are the best methods to keep friction low. Operating loads also affects friction (higher loads result in higher friction).
Stiffness – Includes stiffness (or rigidity) of mechanism components, the mounting brackets and attachment structure. Stiffness affect gearing ratio and can also affect total travel. Cable stretch will have a significant impact on stiffness characteristics. Stiffness should be included in gear ratio computations.
Clearances – Clearance of all mechanism components to other components and structure throughout the full range of motion is critical. Normally some margin is used, such as maintaining 0.25 inch clearance to structure and other components. In areas where structure is rigid, clearances below 0.25 inch can usually be tolerated.
Environmental Considerations – All environmental qualification categories apply to mechanisms. Key categories are temperature, vibration, icing, and fluids susceptibility. The relevant industry guidelines are RTCA DO-160 and Mil-Std-810.
Freeplay/Backlash – Any mechanism will have some amount of freeplay or backlash in the system. Obviously freeplay and backlash should be kept very small. Depending on the function of the mechanism, freeplay may be very critical. In freeplay/backlash is critical, tight tolerance parts, zero backlash gears, etc. should be used to keep freeplay/backlash to a minimum.
Bearings – Mechanisms typically use bearings in pushrods, bellcranks and other components. Bearings come in many different designs (ball bearings, roller bearings, thrust bearings, etc.) and are manufactured from a variety of materials. Selection of high performance bearings with good reliability is important. Military quality bearings are a good choice. Spherical bearings are also normally chosen to prevent binding during mechanism movement. The bearings should be rated for the expected loads (both static and fatigue) to be seen in service. High quality bearings, such as military or aerospace grade should be used in critical applications. Appropriate environmental tests and endurance tests should be conducted to validate the bearing over the expected life in the mechanism.
Weight – Weight should also be minimized to the extent possible. Weight efficiency is always important. During the Design Concepts phase, the mechanism design(s) should be assessed for weight efficiency. Mechanism concept, mechanism simplicity, loads and selection of materials are primary drivers of weight.
Efficiency – Efficiency is determined by dividing the power out by the power input to the mechanism. Power losses occur from friction and compliance of structure and components. If cables are used, cable stretch will also absorb energy through friction between the cable strands (strands rub together as the cable is stretched).
Complexity/Number of Components – As the number of components increases in a complexity and cost increase. Reliability will also decrease. Optimum designs in terms of simplicity and weight are always desirable. During the Design Concepts phase, the mechanism design(s) should be assessed for complexity. Simpler designs, which meet the design requirements, should be selected.
Failure Modes - Failure modes should always be considered in any design. The two most important failure modes are a jam in the mechanism and a disconnection of a component in the mechanism. Another important failure mode is high friction levels – high friction could lead to pilot induced oscillations in a flight control system.
Materials – Materials must be capable of handling limit and ultimate loads plus the endurance/fatigue load spectrum. Manufacturing processes for critical (primary load path) parts should be examined closely to ensure parts will be manufactured with the desired material properties intact. Materials should also be chosen that have well-established material properties. Special attention should be given to new or unique materials. Non-critical (non primary load path) mechanism parts may not require as much scrutiny as critical parts.
Component Design – Design details for each component in a mechanism should be examined carefully. Parts can have (and often do have) a unique shape driven by requirements to carry the necessary loads, provide adequate clearance between structure and other moving parts, and to minimize weight. Some parts may require tight tolerances. Methods of staking bearings are important. Pushrod end clevises must be threaded sufficiently in the rod so sufficient threads are available to carry the load. Cable selection and installation characteristics must also be done properly. Standards exist that cover many detail design features for mechanism components. Military and Aerospace Standards or existing company standards should be followed whenever possible.
Qualification testing for mechanisms should include all of the mechanical environmental tests required by RTCA/DO-160 or Mil-Std-810 or other appropriate environmental specification. Normally environmental tests will be done at component level. For example, bearings and cables can be evaluated separately. Analysis can be used in lieu of some tests. For example, fungus resistance for metal parts can normally be done by analysis. For some mechanisms, environmental qualification will rely on service history of similar mechanisms used in other flight vehicles. Use of service history will reduce the amount of qualification testing that would be required. However, endurance tests and load tests should always be done on new mechanism installations. When conducting endurance and loads tests, the use of actual mounted hardware (brackets and such) is preferred.