Motion Safety Guide for Product Design: Identifying Risks & Solutions

Anywhere human hands are involved, there’s risk.

A grill lid may seem like a simple, open-and-close design. But what about the weight of the lid, the heat underneath it, the location of the user’s hands, and what happens if the lid falls unexpectedly? Or, consider a piece of medical equipment where someone reaches beneath a cover to retrieve vials. If the cover falls or shifts unexpectedly, it could create a safety hazard or cause the user to jerk their hand away while handling something delicate.

So, how does the designer fix or avoid this issue? For starters, safe motion is a behavior you engineer, not a component you buy.

Not all motion safety issues are alike. Preventing a lid from slamming may require a very different approach than reducing the strain of lifting it dozens of times per shift. Solving one problem can even introduce a new one.

Let’s look at how to identify motion safety risks and engineer a solution without losing sight of usability, aesthetics, or cost.

The 2 Big Safety Risks in Motion Control

In evaluating a safety-related design problem, your manufacturing consultant will likely start by asking what types of injuries are occurring. From there, its engineers will try to sort out:

• What action caused it?
• How do we control that?
• Do we eliminate the pinch point, and/or control the force?

The answers to #2 and #3 depend on the action that caused the issue. There are two main ways that motion-based product features can become a safety hazard:

Slamming Lids

Never underestimate the power of gravity. When a heavy lid or cover starts moving, it can accelerate under its own weight. And it can arrive at the end of its range of travel with serious force.

For the user, that creates immediate pinch and crush hazards, particularly around hands and fingers. Generally, the heavier the lid and the less controlled its motion, the greater the potential hazard.

Slamming can also become a product-safety issue beyond immediate injury. Repeated impacts can damage hinges, latches, fasteners, seals, and the lid itself, creating new failure points over the product’s service life. A well-designed motion control system manages the lid’s movement so the user remains in control rather than allowing gravity to call the shots.

Fatigue

Fatigue is the more-overlooked cousin of the previous issue.

Imagine you’re designing a counter space for a bar and restaurant. If a petite, 17-year-old waitress has to lift a 100 lb. flip-up countertop dozens of times per shift, that’s its own kind of hazard:

Whether it’s on a grill or a countertop pass-through, poor motion ergonomics can cause longer-term issues than a simple slamming injury.

How to Choose a Safe Motion Control Solution

So you’ve identified the safety problem. Now, how do you solve it?

With a soft-close feature. A lid stay? Heavier-duty support? A counterbalance?

1. Start With the Scary Behavior, Not the Component

Before somebody says “We need a gas spring!” or “Put a damper on it!”, define what needs to change:

• Is the lid falling too quickly?
• Does it need to stay open so someone's hands can safely enter the workspace?
• Is it simply too heavy to lift repeatedly?
• Does it need to hold at any angle?
• Is the opening force acceptable but the closing force/speed isn't?
• Could the user accidentally release it while their hands are in a pinch zone?

Don't begin by choosing a mechanism. Begin by defining the motion you need to make safer.

2. Separate ‘Controlling Motion’ From ‘Reducing Effort’

Solving a slamming lid is about controlling the movement. Solving for user fatigue is about controlling the effort.

Sometimes a design needs both.

There’s not one particular technology that’s universally appropriate. That’s why you need to translate your safety problem into a motion requirement rather than a simple hardware purchase.

3. Then introduce the variables that determine the actual solution

Once you’ve defined the motion and safety outcome you need, then you can go back and toy around with the “middleman” – the engineering inputs. Key variables to evaluate include:

• Lid/door weight
• Center of gravity
• Dimensions & geometry
• Range of motion
• Desired opening/closing force
• Frequency of use and expected cycles
• Available mounting space
• Pinch points
• Operating environment (temperature, moisture, contamination, etc.)

Don't evaluate those factors in isolation. Changing how a lid moves can change how the user interacts with the entire product.

Can Adding Safer Motion Create a Trade-Off?

Making motion safer usually means controlling it more. Additional control can come at the cost of product feel, visuals, and cost itself, not to mention the ease of getting it out the door in the first place.

Safety vs. Aesthetics

Whether it's a counterbalance, linkage, damper, guard, or some other mechanism, your safety hardware has to physically go somewhere. Accommodating a larger mechanism may require a bulkier enclosure or lid geometry. Protecting users from pinch points may require linkage covers or guards.

This may hurt your heart if you're a designer, but an elegant concept can become a bit chunkier and clunkier once safety requirements enter the mix. Safety is, of course, more important than aesthetics, but your engineering team should work toward the goal of satisfying both.

Safety vs. User Experience

A safer design can also change how the product feels to operate. You can create something safer technically while also making it annoying to use.

For example:

• Too much friction → lid becomes difficult to operate
• Too much lift assistance → lid opens too aggressively or is too difficult to close
• Excessive damping → user has to wait too long for a lid to move
• Awkwardly positioned lid stay → inconvenient to engage/release
• Extra guarding → may interfere with access

Your mission is to provide enough control to address the hazard without introducing undesirable motion or inconvenience somewhere else in the operating range.

Safety vs. Complexity

Every extra mechanism potentially adds:

• Assembly steps
• Mounting requirements
• Failure points
• Maintenance

That doesn't mean "simpler is safer." Quite the opposite, sometimes. But it means you should ask whether the safety improvement justifies the additional complexity and whether there's a simpler mechanical solution that can achieve the same result.

Safety vs. Cost

The teams in your company with a pulse on budget are looking for you to provide the best result for the least amount of money. However, the cheapest motion-control solution isn't necessarily the lowest-cost solution over the life of the product.

Beyond the initial component cost, you've potentially got these to worry about:

• Engineering/redesign cost
• Assembly cost
• Durability/cycle life
• Warranty/service issues
• Potential injuries
• Workers' compensation/liability exposure
• Downtime/productivity effects

Always weigh your initial investment against the consequences of a low-cycle or dangerous product.

Tips for Evaluating Motion Safety

What's the #1 rule for evaluating safety before a product hits prototyping or testing? We couldn't settle on one, so we broke the rules and picked two:

Tip #1: Do a DFMEA

Sometimes the biggest failure mode is the one you didn't think of or didn't take seriously. For that reason, we highly recommend doing a design failure mode and effects analysis (DFMEA).

A design FEA can help you:

• Identify motion-related failure modes you may not have considered
• Evaluate the potential safety impact of each failure
• Prioritize the risks that deserve design changes first
• Challenge assumptions

Your component vendor can initiate the analysis, but your participation is important. Manufacturing, engineering, quality, and other departments create a more diverse focus group and a better result.

Tip #2: Engineering Support Is There – Use It

While your engineering and design teams know your product, a third-party support partner can add valuable insight on the technical nitty-gritty.

The biggest benefits of outside engineering assistance are:

• Identifying hazards the OEM team may overlook: When you've spent months designing a product, you start making assumptions. A motion-control specialist can look specifically for uncontrolled movement, pinch and crush points, excessive operating forces, unexpected movement, and potential failure modes.
• Matching the mechanism to the safety problem: An outside expert can best determine whether the application calls for damping, friction, counterbalancing, a hold-open feature, or some combination.
• Correct calculations: Specialized motion engineers grasp not just weight, but also lid geometry, center of gravity, hinge position, operating angle, desired user force, etc.

The sooner you get engineering experts involved, the better. Some of the trade-offs you learned about above (like aesthetic impact) are much easier to address early in product development. The grill lid example is Exhibit A: The designer had achieved the desired motion, but the project had progressed too far to accommodate the safety problem without a major redesign.

Engineering a Balance

The objective of motion safety isn't to eliminate every design trade-off. It's to understand them early enough that safety, usability, aesthetics, and cost can be engineered together rather than forcing one quality to compensate for another.

When evaluating product safety, start with how the injury is happening, and work backward from there. To have an expert help you with the diagnosis process, drop a question here: