Constant-force springs
All images courtesy John Evans’ Sons

Engineers have several options to counterbalance heavy forces in motion-control mechanisms for aerospace equipment. Traditionally, gas springs or helical springs have secured aircraft seating, entry doors, and cargo doors. However, counterbalance alternatives including constant-force springs, spiral-torsion springs, and spring reels require less space, provide greater flexibility, and offer better longevity. Here are some pros and cons of each.

Constant-force springs

With smooth, consistent motion coupled with a constant load, constant-force spring aerospace applications include aircraft doorways, overhead bins, and interior and exterior motion control mechanisms. Whether they retract or extend, constant-force springs maintain a constant load. Another benefit is that users can increase the load capacity by using multiple constant-force springs in a gang arrangement.

As with other springs, extreme temperatures and some mounting methods can affect performance.

Spiral-torsion springs

Spiral-torsion springs

For parts requiring a greater range of motion, spiral-torsion springs can safely extend 270°, and can be designed for a small space. Their range of motion and efficiency also provides flexibility. This component may only require small deflection to achieve the necessary amount of counterbalance.

The main disadvantage is that spiral-torsion springs don’t work as well for applications that require a 360° range of movement.

Spring reels

For moving parts, spring reels can counterbalance loads heavier than 50 lb and can reach 10ft of deflection. They primarily help in retracting doors, steps, and hatches.

Spring reels (pictured, pg. 16) require less energy to operate than other spring counterparts, and compact reels can be used in tight spaces.

Spring reels

Other considerations

Gas springs are often more cost-effective upfront, but design engineers should consider other counterbalancing mechanisms because of:

  • Lifespan – The cost of counterbalance mechanisms versus gas springs reflects the maintenance required. Gas springs deteriorate throughout time and become less likely to perform well. In contrast, counterbalance springs can last 250,000 cycles, a long service life, even in commercial applications where aircraft cargo doors open and close multiple times in a 24-hour period.
  • Flexibility – One major downside of gas springs is that users cannot make adjustments if devices start to fail. The choice is putting up with subpar performance or replacing the entire system. If the cost is $60 compared to $1,000, this might not seem like much, but in commercial settings that include multiple pieces of equipment, the cost can be significant. Engineers can adjust counterbalancing equipment if or when it begins to show wear. This reduces the need for a replacement and leads to longevity. It also makes the counterbalance option ideal for applications such as outer space, as counterbalance systems need very little maintenance.
  • Resilience – Counterbalancing technology stands up better to extreme heat, cold, and pressure, making it more suitable for outer space use. Many companies use counterbalance mechanisms for aircraft systems that are exposed to extreme external conditions.
  • Safety – Counterbalancing equipment avoids the risk of explosion, which can happen when gas springs face extreme pressure. This puts lives at stake, and once engineers discover the risk, correcting the problem might require a complete redesign. Hinge solutions and other alternatives might also fail to safely handle heavier loads.

Overall safety is why so many companies now rely on counterbalance doors and hatches. The lower failure rate due to explosions or year-on-year deterioration means that operators can continue to safely lift, open, and close heavy doors with general ease, potentially for decades. Additionally, it is easier to disinfect areas around counterbalance systems because they are so compact.

Bottom line

While gas springs have been used in the aerospace industry for years, mechanical springs often provide superior performance. Constant technological advancements and demands put on aerospace equipment make it worth considering if counterbalancing components other than gas springs are a better long-term investment for designs.

Regardless of which option chosen, make sure to work with an AS9100-certified spring manufacturer to ensure that components meet the strictest quality standards in the aerospace industry.

John Evans’ Sons

About the author: Edward Jones is the marketing manager of John Evans’ Sons, the AS9100-certified, ITAR-registered international designer and manufacturer of constant force springs, constant force spring assemblies, spring powered reels, helical vibration isolators, and other mechanical components.