Anti-cage creep (ACC) technology prevents cage drift in linear guides such as ball guides, cross roller guides, and needle roller guides. In more casual contexts, sometimes called anti-friction linear guides. Even with high accelerations of up to 15G and high-frequency short-stroke movements, ACC has been proven to be the most compact, robust, and reliable solution available in the market. Read here about our solution.
Precision linear guides, often called cross roller bearings or caged linear guides, play a crucial role in various positioning systems. One set of linear guide consists of two pair of guides where one pair include two rails with a cage with rolling elements in between. However, the phenomenon known as "cage creep" can impact their performance. In this article, we'll delve into the concept of cage creep, explore the need for anti-cage creep solutions, and highlight the unique features of PM's Anti-Cage Creep (ACC) system.
Cage creep can occur in linear guides with limited strokes and cages. Typically non-recirculating linear guide applications where vibrations, improper mounting, very high acceleration (and de-acceleration), inadequate tolerances on the mounting surfaces, and uneven preloading, or moment loading are present. Vertical applications are also sensitive to cage slippage, simply due to the force of gravity where the cages slowly creep down.
A linear guide consists of two guiderails and a cage fitted with rolling elements (balls, cylindrical rollers, or needle rollers). In this linear guide, the cage is positioned in the middle of the rail (image 1).
A linear guide has a certain maximum stroke. At this maximum stroke, the cage is just between the rails. It is held back by end screws or end plates (image 2).
A linear guide can be used in a system with, for example, high accelerations. During these high accelerations, the cage will slowly shift. This shifting is called creep. The result is that the stroke becomes smaller and smaller. As the cage drifts from its original position, increased friction, reduced travel length, and premature wear of the linear guides occur. By cage creep the cage will hit the end stops and can damage the cage (image 3). This results in a shorter lifetime and can result in premature failures of the linear guide.
To recover the maximum stroke, the cage has to be reset. For resetting, the end screw or plate at the end of the rail is used. During reset, the cage is pushed back to the middle of the rail using the end screw. If there is a lot of pre-tension on the set, a high force is needed for resetting. The force for the reset comes from the driving motor.
Cage creep in linear guides is a undesired movement or slipping of the cage from its original position between the guide rails. This results in a shorter effective stroke length of the linear guide.
The factors contributing to cage creep in linear guides include:
High accelerations: Rapid accelerations can create forces that may cause the cage to slip from its original position.
Influence of gravity during Z-movements: When the linear guide is involved in vertical (Z-axis) movements, the influence of gravity can contribute to cage creep.
Misalignment: If the linear guide components are not properly aligned, it can lead to uneven forces on the cage, causing it to move unintentionally.
Poor Adjustment of preload: Preload is the force applied to eliminate clearance between rolling elements. If the preload is not correctly adjusted, it can result in cage creep.
Addressing and minimizing cage creep is crucial for maintaining the accuracy and reliability of linear guide systems.
Cage creep can result in damage of the linear guides and unexpected downtime of the machine.
Here are some tips to prevent cage creep and ensuring optimal performance:
Proper finishing of the mounting surfaces of the guide rails,
Proper installation (link to installation guidelines of linear guides | step-by step),
Good alignment of the rails, within the specified requirements (reference Installation guide)
Precise adjustment of preload
Correct lubrication and regular monitoring
When considering the need for an anti-cage creep solution in linear guides, several scenarios may hinder the conventional cage reset outlined in the preceding section. The following are situations where standard reset procedures may not be feasible:
Insufficient cage strength: the cage in use may lack the necessary strength to resist pushing back to its standard position.
Infeasible reset force: the required reset force may exceed the capabilities of the motor employed in the system.
Space constraints in machine cycle: the machine's cycle may not accommodate the necessary room or space for a reset stroke.
Out-running cages: the presence of out-running cages can impede the straightforward resetting of the cage.
To address these challenges and enable the continued use of a set within a system, opting for a set with anti-cage creep (ACC) is recommended.
Applications of a set with ACC are particularly advantageous in vertical setups and high-acceleration environments. The absence of cage creep ensures reliability, making it an ideal solution in scenarios demanding high repeatability. The ACC feature guarantees that the rollers consistently return to the same position, maintaining the original adjustment at that specific location.
In situations were a shorter slide moves over a longer rail linear guides with outrunning cages are used. PM supplies linear guide sets where one of the two guiderails in a pair is shorter than the cage. As the cage is longer than the shorter rail these rail ends are without end pieces and supplied with rounded inlets. These inlets will help the cage to move-in or move-out of the pre-load zone of the rails more smoothly.
Not all cages are suitable for outrunning. It depends on cage characteristics such as cage material, cage length, and rail geometry. This all needs to be considered.
The advantage of outrunning linear bearings is that you need less installation space for the same stroke.
In the provided image, it's evident that only one of the two rails extends beyond the cage. Despite using outrunning cages, there is a tendency for the cage to gradually shift over time. To perform a cage reset, the ability to exert force on the cage using both rails and the end screws is essential. However, the current setup lacks enclosure on one side, making a hard reset unfeasible.
Consequently, it's not possible to reset a cage in a configuration where the cages protrude beyond the rails.
The concept of "Anti-Cage Creep" (ACC) refers to the technique of preventing any slipping of the cage that holds the rolling elements between the two V-groove rails of the slide way. By ensuring an accurate movement and eliminating creep creep, maintenance costs are reduced, precision is increased, and downtime is minimized.
PM engineers have further enhanced the anti-cage creep technology (ACC), making it suitable for high-tech and extremely dynamic applications. ACC is integrated into cross-roller linear guides either with a glued rack or Electro Chemical Machining (ECM) directly into the rail. Anti-cage creep is available in two versions:
1. ACC: with glued rack from brass in the bottom of the V-groove
2. ACCI: embedded rack machined in the bottom of the V-groove and through hardened. The length of the rail is limited by the machine. This version is only available with stainless steel linear guides. The (ACCI) anti-cage creep solution is popular with applications in vacuum and ultra-high vacuum environments.
For decades, the anti-cage creep solution of PM has proven its superior ability to prevent cage creep when applied in the most demanding applications and operating in the most challenging conditions.
Within PM, an ACC based on a gear rack transmission according to DIN 867 is used. This gear rack is designed for continuous power transmission. The rack sits in the clearance (bottom of the V-groove) of the rail, the gear is part of the cage assembly.
The gear rack is placed internally in the set. The height and width of a set with ACC is the same as a set without ACC. The advantage is that this makes the sets interchangeable without adding extra costs for engineering and machining In case of failure, the sets can be easily swapped.
There are two versions of anti-creep systems within PM. In the standard type, a brass rack is glued into the bottom of the V-groove, this is called ACC.
Another solution is to integrate the rack into a stainless steel rail using ECM technology, this is called ACCI. The advantage of ACCI is that the rack is directly in the material, instead of having a glued connection which makes it more robust and suitable for applications in special environments as ultra-high vacuum.
The ACC technology is integrated into the design of the linear guide without influencing the external boundary or mounting dimensions. Linear guides with anti-cage creep solution have the same attachment holes as the standard linear guides. This allows for drop-in replacement of guides in under-performing applications with the anti-cage creep solution without additional machining or redesign costs for table parts.
We offer two types of linear bearing models which are available with anti-cage creep technology ACC;
RSDE-ACC high load cross roller guide, available in roller sizes 3, 4, and 6 mm. Drop-in solution for the standard precision linear guides type RSD
RNG-ACC, space and weight saving cross roller guide, available in roller sizes 4 and 6 mm.
The V-groove geometry offer for both rail types a longer surface contact area. In combination with the tightly spaced cylindrical rollers in the cage, the running features are unbeatable.
There are two types of anti-cage creep cages in linear bearings. The cages can be used for both linear guide types, RSDE and RNG rail guides. For normal usage cage type, KRE can be selected, made of polymer (POM material) within the center a small metal spur gear that matches with the gear rack integrated into the rail.
For vacuum and ultra-high vacuum environments cage type KREV, made from PEEK is available in roller sizes 4 and 6 mm.
These roller cages can optionally be supplied with stainless steel cylindrical rollers.
The ACC solution for linear bearings is the result of an extremely careful design and manufacturing process. The graph illustrates the resulting fact that the force to be applied to overcome friction remains virtually unchanged.
Linear guides with ACC technology are capable of operating in temperatures up to +80 °C. This gives ACC a significant advantage over similar systems using plastic components.
Max. acceleration 150 m/s² (15G)
Anti-cage creep linear bearings are popular in use. Attached is a summary of the main benefits cited by our customers as to why they chose PM's solution.
A robust and long-lasting solution
No redesign cost by replacement of standard linear guides
Low additional costs compared to a set of standard guides
Smooth, high-accurate motion
Does not affect the load-carrying capacity
A reliable solution in every mounting orientation
A set anti-cage creep cross-roller linear bearing consists of four rails and two roller cage assemblies. End pieces are for cage stop not required since the roller cage can not move out of the rails as it is controlled by the rack and gear system.
We will dive more deeply into topics that are related to the performance of linear guides and the impact of anti-cage creep on it.
The impact of Anti-Cage Creep (ACC) on functionality is a critical aspect that underwent thorough examination during the design phase. Rigorous research was conducted to assess its effects on key performance parameters, including running resistance, load capacity, and stiffness. This section delves into the findings of this comprehensive analysis, shedding light on how ACC influences these crucial aspects of linear bearing systems. Understanding the implications of ACC on running resistance, load-bearing capabilities, and stiffness is paramount for engineers and designers aiming to optimize the performance and reliability of their systems.
The influence of Anti-Cage Creep (ACC) on friction resistance, especially in the context of rack design, is a crucial consideration for technical engineers. The rack is carefully manufactured to ensure smooth rolling of the gear along its entire length. The manufacturing process follows strict tolerances so that the teeth engage seamlessly and silently with the tooth profile. Extensive running resistance studies show that, remarkably, there is no measurable difference between a cage with ACC and one without. This finding highlights the effectiveness of ACC in limiting cage creep without introducing additional friction problems in rack and pinion applications and provides engineers with valuable insights for optimal system design.
Inserting the gear into the cage involves utilizing two rolling elements, inevitably impacting the load
capacity compared to a cage without the gear. This impact is more significant in sets with fewer rollers than those with a greater number of rollers.
For example, a linear guide set without Anti-Cage Creep (ACC) has a higher dynamic load capacity than a comparable set equipped with ACC, as shown above. The difference between the two sets is the number of rollers - 22 for the ACC-cage and 24 for the standard cage. Removing two rollers from the original set results in a significant 7% reduction in dynamic load capacity, providing engineers with valuable insights into the load-bearing implications of integrating ACC in different linear guide configurations.
If a set without ACC is replaced by a set with ACC, the load capacity reductions mentioned above will have to be added up. As an example, an RSDE-3125x26KRE is replaced by an RSDE-3125x24KRE-ACC. As visible in the figure below, the dynamic load capacity for this set will decrease by 13%. The outer dimensions and stroke, of 76 mm, will remain the same.
The stiffness of the cage, primarily dictated by the outer rollers, remains a critical consideration for technical engineers. Introducing a gear into the cage has no incremental impact on stiffness reduction beyond that resulting from the removal of two rollers. This is attributed to the gear's central placement within the cage, a visual representation of which is provided in the figure below.
When transitioning from an RSDE-3125x26KRE to an RSDE-3125x24KRE-ACC, maintaining stroke lengths and outer dimensions, the stiffness of the set experiences a 10% reduction. Similarly, removing two rollers from a set (RSDE-3125x24KRE-ACC to RSDE-3125x22KRE-ACC) results in a comparable 10% decrease in stiffness. Notably, accommodating ACC necessitates lowering the bottom of the V-groove, an adjustment that, as indicated by the results, has a negligible impact on set stiffness.
These findings offer insights into the nuanced relationship between gear integration, roller removal, and the resulting stiffness characteristics in linear guide systems.
The gear rack system implementation brings about more complexities in a set, which causes worries for technical engineers. Unlike a set without Anti-Cage Creep (ACC), the inclusion of a gear rack increases the chances of particles getting stuck.
The design of the gear rack system cleverly includes enough space to effectively trap particles. This is made possible by the cage filling the gap between the rails, creating a nearly seamless seal. As a result, this design reduces the risk of contamination, alleviating engineers' concerns by creating a protective barrier against unwanted particles in the system.
However, engineers must keep in mind that a sealed linear bearing offers the best protection against particles but has also some negative effects as a higher friction resistance.
A set with a glued brass rack (which is the standard) is basically self-lubricating. For a set with glued racks, the same lubrication can be used as a set without ACC. However, it's important to avoid using too much lubrication, as this can cause the rack to push it aside. By keeping lubrication between the rack and gear, particles are less likely to get between the rollers and tread, reducing the risk of damage.
A set with stainless steel rails and integrated rack, called ACCI, needs lubrication. Both the rack and the gear are made of stainless steel. For long-lasting the running surfaces of the rails, rack, and gear need to be lubricated. The rails and rack are through-hardened en the gear is made of stainless steel AISI 304. This makes the gear more sensitive against wear by insufficient lubrication.
This link provides more information about the lubrication of linear guides
For UHV applications there are two options for anti-cage creep:
Glued racks with UHV-compatible plating
Integrated racks type ACCI
There are several cage designs available to use linear bearings in ultra-high vacuum environments.
For support on the right selection contact the engineers at info@PM.nl
Glued racks with UHV-compatible plating. One generally accepted anti-cage creep solution for linear bearings is with glued and plated racks. The racks are gold-plated and glued inside the cleaned rails. This solution is used for rails with lengths over 250 mm.
Integrated racks type ACCI. For guide rails with lengths shorter than 250 mm, it is possible to integrate the racks in the rails. They are machined in the bottom of the V-groove without affecting the load capacity. This is a robust solutions and long-lasting solution and is free of glue or other additives.
Anti-Cage Creep (ACC) may be deemed unnecessary under specific conditions where the system possesses inherent capabilities to autonomously reset the cage. Key parameters influencing this determination include:
Low movement speed: Systems characterized by low set movement speeds may inherently manage cage resetting without the need for ACC.
Low roller preload: If the preload on rollers is low, the system may exhibit a reduced tendency for cage creep, obviating the necessity for ACC.
Low accuracy and repeatability: Systems with lower precision requirements, both in terms of accuracy and repeatability, may not necessitate ACC support.
Engaging with a PM engineer is recommended to explore customised solutions and assess the viability of ACC based on specific system dynamics and requirements
A high amount of applications that use anti- cage creep linear bearings can be found in short stroke (< 125 mm) and high acceleration applications.
These are semiconductor and industrial-related applications like pick-and-place, inspection, and actuators, often used 24h/day.
Other application which require extremely high levels of reliability and repeatable precision are medical technology, life science scanning, and laboratory instruments.
Miniature linear slides type MSR include anti-cage creep technology ACCI. They are available in 7 sizes and several lengths. All of them are made of stainless steel.
In summary, Anti-Cage Creep (ACC) has a significant impact on precision linear guides, solving challenges such as cage creep. PM's ACC system, which uniquely uses a rack and gear, ensures non-noticeable, virtually frictionless operation and offers optimum interchangeability with other caged linear guides. This allows for significant cost savings. Anti-cage creep solutions offer high reliability in high-performance applications with repeatable precision accuracies.
New insights into friction resistance, contamination risks, lubrication, and the possible applications in ultra-high vacuum make ACC a solution that can be used for many. ACC's negligible impact on performance underlines its relevance. For scenarios where ACC may not be needed, key parameters guide engineers. Collaboration with PM engineers is recommended for customised solutions, providing a holistic understanding of ACC's role in optimising linear guidance systems.