Linear Actuator Sizing: Load & Speed

1. Understanding Linear Motion Systems

Before we can define requirements, we need a shared understanding of what a linear motion system is. At its heart, it's a system that converts energy (usually electrical) into motion along a straight line. These systems are everywhere, from 3D printers and CNC machines to automated factory equipment and adjustable desks.

To get a broad overview of the technology, its applications, and the key terminology, let's start with a short video.

As the video shows, a linear motion system is more than just a motor; it's a combination of a motor, a transmission mechanism (like a lead screw or a belt), and guidance components (like rails and bearings). Our job as design engineers is to select or design these components to meet a specific goal.

2. The Key Requirements for a Linear Actuator

To select the right components, we need to translate a general goal (e.g., "lift a box") into a precise set of engineering specifications. A popular industry acronym for remembering the key parameters is LOSTPED:

• Load
• Orientation
• Speed
• Travel
• Precision
• Environment
• Duty Cycle

Let's break down each of these. As we go, we'll draw on your mechanical engineering background to understand the physics behind these requirements.

Load (Force)

This is the force the actuator must exert. It's not just the weight of the object being moved. You must also account for:

• Friction: Force resisting motion from guides, seals, etc.
• Acceleration Forces: The force needed to get the mass moving, based on Newton's second law ($F=ma$).

Given your background, you'll recognize that these forces translate into torque requirements for the motor. The friction force results in a constant friction torque, while the acceleration force results in an acceleration torque.

Crucially, always apply a safety factor. Real-world conditions are never perfect. Friction can be higher than calculated, components can misalign, and materials can vary. A typical safety factor is 1.5 to 2.0, meaning you select an actuator rated for 1.5x to 2.0x your calculated required force.

Speed

This is how fast the load needs to move, typically measured in mm/s or inches/s. Speed is fundamentally linked to force and the actuator's internal gearing and screw pitch. This gives rise to the primary tradeoff in actuator selection:
High force typically means low speed, and high speed typically means low force.

When defining speed, you are defining the motion profile. For most applications, we assume a trapezoidal profile: the actuator accelerates to a constant speed, travels, and then decelerates to a stop.

Travel (Stroke Length)

This is the total distance the actuator needs to move the load from its start point to its end point. It's important to distinguish this from the physical dimensions of the actuator itself. An actuator with a 300 mm stroke will be much longer than 300 mm when fully extended.

Duty Cycle

This defines how often the actuator is working. It's usually expressed as a percentage of time the actuator can be "ON" within a given window (e.g., 20% duty cycle means 2 minutes on, 8 minutes off). Exceeding the duty cycle causes the motor to overheat, drastically reducing its lifespan.

Orientation

This refers to how the actuator is mounted and the direction of the load relative to the actuator. Is it pushing horizontally? Lifting vertically? Opening a hinged lid? The orientation determines which forces are dominant (e.g., gravity in a vertical lift) and is critical for calculating the true load.

Precision (Accuracy & Repeatability)

• Accuracy: How close the actuator can get to a commanded position.
• Repeatability: How consistently the actuator can return to the same position.

For an application like a CNC machine, these are critical. For lifting a toolbox, they are not. Over-specifying precision is a common and expensive mistake.

Environment

This covers the operating conditions:

• Temperature: Will it be used in a freezer or a furnace?
• Contaminants: Will it be exposed to dust, water, or corrosive chemicals?
• The IP (Ingress Protection) rating of an actuator tells you its level of sealing against solids and liquids.

3. Defining Requirements for Project 1

Now, let's apply this framework. For our first project, we will design a motorized vertical lifting system for a home workshop.

Project Goal: Design a system to lift a standard toolbox off a workbench to an overhead shelf for storage.

Let's translate this goal into a formal requirements document.

1. Define the Key Variables:

• Load Mass: A reasonably heavy toolbox might weigh 15 kg.
• Travel Distance: The distance from the workbench to the shelf is 600 mm.
• Desired Speed: It shouldn't be too fast (unsafe) or too slow (annoying). Let's aim to complete the lift in 10 seconds.

2. Create the Requirements List:

Now we fill in our LOSTPED/7-criteria checklist:

• Load (Force):
  • Static Load (Weight) = Mass × g = 15 kg × 9.81 m/s² = 147.15 N.
  • Let's apply a safety factor of 2.0 to account for friction, acceleration, and other unknowns.
  • Required Force = 147.15 N × 2.0 ≈ 300 N (Dynamic). The actuator must also be able to hold the 147 N load statically without back-driving.
• Orientation:
  • Vertical lift. The load is primarily axial tension/compression on the actuator.
• Speed:
  • Average Speed = Distance / Time = 600 mm / 10 s = 60 mm/s.
• Travel (Stroke):
  • The actuator must move the full distance. Stroke = 600 mm.
• Precision:
  • High precision is not required. The user can visually confirm the final position. Let's define it as +/- 5 mm repeatability.
• Environment:
  • Indoor home workshop. Generally dry, room temperature. Some potential for sawdust. A basic IP54 rating would be sufficient.
• Duty Cycle:
  • The lift will be used infrequently, maybe a few times a day. A low duty cycle is acceptable. 10-20% is sufficient.

Here is what our preliminary requirements document looks like:

This table is now our guiding document. Every component we select must meet or exceed these specifications. Tools like the one shown in the image below use exactly these parameters to help you find suitable products.

Conclusion

In this lesson, you've taken the first and most important step in our project: defining the system requirements. This process transforms a vague goal into a concrete, actionable set of engineering specifications.

Key Takeaways:

• A design project must begin with a clear requirements document.
• The LOSTPED framework (Load, Orientation, Speed, Travel, Precision, Environment, Duty Cycle) provides a systematic way to define requirements for a linear motion system.
• Load and Speed are primary drivers, but they are part of a larger system of interdependent parameters. Understanding their tradeoffs is key to good design.
• Always include a safety factor to account for real-world uncertainties.

You have now created the foundation for Project 1. This requirements document will be the "contract" for our design.

In the next lesson, we will use these exact requirements to perform sizing calculations for a lead screw or ball screw. You will see how the force, speed, and travel specifications directly influence the selection of the core mechanical components.