How do I start adding chains and pulleys to my forklift model and why don't all models have chains?

First hello, second it would help to know which software you are using. Most Cad software don't have built in addins that animation these things as it take a lot of processing power, however it is possible if you know how to use your Cad software.

Here's a step-by-step guide on how to begin adding chains and pulleys to your forklift model in your CAD software (I'll assume you're using a parametric modeler like SolidWorks or Inventor, but the process is similar in other CAD packages):

Reference Images and Measurements:

Gather reference images or blueprints of real forklifts, paying close attention to the chain and pulley systems.
If you have access to a real forklift, take measurements of the chain links, pulley diameters, and distances between components.
Sketching:

Start by sketching the paths that the chains will follow. These paths should accurately represent the movement of the forks and any other mechanisms that use chains.
Next, sketch the pulley locations along these paths. Make sure to consider the pulley sizes and their orientations relative to the chain paths.
3D Modeling:

Chains:
Use the "chain" or "link" feature in your CAD software to create the chain links. You'll likely need to specify the link size, pitch, and number of links.
Some software may have pre-built chain components, which can save time.
Pulleys:
Create the pulleys using basic revolve or extrude features. Ensure the pulley diameters and groove shapes match your reference data.
Assembly:
Assemble the chains and pulleys, ensuring they are correctly aligned with the sketched paths.
Use constraints or mates to define the relationships between components, allowing for realistic movement and simulation.
Simulation and Refinement:

If your CAD software allows, perform motion simulations to verify that the chains and pulleys move as intended.
Make any necessary adjustments to the design based on simulation results or further analysis.
Additional Tips:

Simplify: Start with a simplified chain representation, and add more detail as needed.
Standard Parts: If possible, use standard chain and pulley components from your software's library or from online resources.
Material Properties: Assign appropriate material properties to the chains and pulleys for accurate simulations and stress analysis.
Consult Experts: If you have access to mechanical engineers or forklift experts, seek their advice for design and functionality considerations.
Why Don't All Models Have Chains?
Not all CAD models include chains (or other detailed components) due to several factors:

Complexity: Chains are complex assemblies with many individual parts. Modeling them in detail can significantly increase file size and computational requirements.
Purpose of the Model: If the model is intended for conceptual design or visualization, detailed chains may not be necessary.
Level of Detail: Some models are intentionally simplified to focus on specific aspects of the design or to reduce file size for easier sharing and collaboration.
Time Constraints: Creating detailed models takes time. Designers may choose to omit chains if they are not critical to the project's goals or deadlines.
In your case, since you are specifically interested in the draw, trim, and bend surfaces of the forklift, adding detailed chains and pulleys will provide valuable context for your surface development work. Just be mindful of the complexity and file size implications, especially if you need to share your model with others.