Imagine a refuse truck operator picking up a dumpster. Hydraulics unload the bin into the back of the truck, flipping the dumpster upside down and shaking the trash free.
Located inside the hydraulic cylinders of those arms are retaining rings. These rings are withstanding a load as they are hammered by the movement of the hydraulic arms. Predicting the intensity of that type of load is a key part of retaining ring design.
For the refuse truck scenario, the ring must be designed to compensate for shock load, which requires a thicker retaining ring to handle the pressure of the application.
In a high speed milling machine with a spindle turning at 25,000-30,000 rpm, however, the design engineer must think of a different solution.
In this scenario, the designer must establish the maximum rotational capacity of a ring. To determine this, the designer must use a formula to determine at what point the ring would centrifuge off the bottom of the groove.
ENGINEERING.com recently had the opportunity to speak with Ben Moskalik, senior research and development engineer at Smalley, to discuss the most important things to consider when designing and using retaining rings.
Rules of Thumb in Retaining Ring Design
“There are some rules of thumb that we use here at Smalley that we can share,” Moskalik said. “The ring being seated in the groove is truly one of the most important aspects of a retaining ring.”
The groove of the ring should never be the same as the ring thickness, or else the ring will never install correctly. If the ring isn’t correctly seated, the ring will easily work itself out of the groove.
“We suggest the groove be wider by a few thousandths of an inch than the thickness of the ring, so we can ensure the installation will be done properly,” Moskalik explained. “However, if the groove is too wide, too much axial load to the outer edge of the ring will cause it to dish over if it isn’t feeding against something.”
The depth of the groove should be about one third of the width of the ring. For example, if you have 0.24 wide ring then the groove at that point would 0.08.
This prevents a lever arm situation where if the groove is too shallowed, the ring could dish out more easily.
If a deeper groove isn’t available, it is acceptable to leave two thirds of the ring sticking out while one third remains in the groove.
In applications where the removal of a ring may be a frequent occurrence, it’s important to design for ease of removal. This could be as simple as including a scallop into the ring for a better grip.
More importantly, it’s vital that design engineers consider how users will treat the ring. If a ring must be removed, will it be replaced? Or will it be used again until it fails? The ring must be designed for durability and be capable of clinging to a shaft or groove even after one or more removals.
“Not everyone is gentle when they take things off,” Moskalik said. “From a design standpoint, if we know that someone is removing it often, we want to make sure our installation stresses are low.”
Finding the right strength material can be tricky. Designers would want the ring to be strong to keep their cling, but not so strong that users would have to deform the ring to remove it.
“We would prefer retaining rings be discarded if they’re going to be removed, but that’s not always the case,” Moskalik added.
External Applications and Retained Components
An external retaining ring is installed along a groove in a shaft. (Image courtesy of Smalley.)
Shaft retaining rings are much more sensitive to installation stresses than bore rings.
Moskalik explains that due to residual stresses built up in the coiling process rings will naturally try to straighten themselves out to their original shapes.
When the ring is wrapped around a shaft, it’s spread out and so it is closer to its original straight shape. Bore rings, on the other hand, will cling to the area they’re housed in for the same reason.
Designers can work around this issue by providing stress relief to the springs around a shaft, and only designing up to 80 percent of the stress that they would for a bore ring.
An inserted internal retaining ring. (Image courtesy of Smalley.)
Remember that retaining rings operate as shoulders – gears and other components are consistently going up against these rings, whether on a shaft or in a bore.
“If there is huge chamfer or gigantic radius on the retained component, it could roll over the ring and dish it because you’re now hitting the ring at a higher point on the outside diameter,” Moskalik said.
“The amount of ring sticking out for a 45° chamfer is 0.375 max chamfer for the amount of the ring sticking out proud of the shaft. Essentially, the overall ring width minus the groove depth and the maximum radius we’d want is half the ring sticking out.”
Overcoming Retaining Ring Failures
If confronted with a retaining ring failure, it may be necessary to go back to the design stages to discover the source of the issues.
For example, if an application required a greater than typical rotational capacity, designers could adjust for greater cling or increase the ring’s width to inhibit that excess of rotational capacity.
Alternatively, designers can also turn to self-locking mechanisms, essentially a tab in a slot. These small tabs on the inside turn, which can lock into a slot on the outside turn.
Self-locking allows the ring to operate at higher rotational capacities while absorbing a degree of impact loading.
“If an application was going to be rotating a ring, whether at 10 rpm or 10,000 rpm, a designer is going to need to know,” Moskalik said. “They don’t necessarily have to specify they need a self-locking ring, but designers should want to make sure that their application requirements aren’t going to exceed what the ring can handle.”
For manufacturers and design engineers who want to work with Smalley to create a custom retaining ring, it’s important to know exactly what they expect from their application to prevent ring failure.
A ring can typically fail in two ways:
- If the groove is the source of the failure, it is commonly due to deformation. This prevents the ring from seating properly in a dished groove that used to be a square groove.
- Alternatively, the ring may shear into two pieces.
The biggest mistake that any design engineer could make when developing a retaining ring is underestimating the importance of the groove in the design process, Moskalik continued.
“Ring failure typically starts at the groove, not the ring…. We build in a safety factor of typically two for groove deformation formulas to make sure the retaining ring isn’t going to pop out. People sometimes make their grooves out of a soft material, or they may not be deep enough or square enough. The integrity of that groove and the material it’s made out of are important things and tend to be overlooked.”
Smalley offers tools on their website to help engineers design more efficient custom retaining rings.
Smalley Steel Ring Company has sponsored this post. They had no editorial input into this post. All opinions are mine. --Kagan Pittman