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.