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Deep groove ball bearing axial load capacity is the first check when a robot axis stands vertical. Gravity pulls the arm, the payload, and the housing straight down the shaft. On a horizontal axis that same mass reads as radial load. Tilt the axis 90 degrees and the load path changes completely. The bearing now carries the force through the ball to raceway contact points that were designed for a rolling motion, not a sliding one. This article follows the axial load path on robot axes: where the loads come from, how to size them against published ratings, and where the limit sits before a design should move to a crossed roller bearing.
Every number below comes from Yuanhe product pages or from ISO load calculation practice. A deep groove ball bearing such as the 6203 bearing lists a dynamic load rating Cr of 9.58 kN and a static rating C0r of 4.78 kN. Those two numbers anchor most of the checks below.
01Where Axial Load Comes From on a Robot Axis
Axial load appears whenever part of the applied force points along the shaft axis. Three situations dominate on robot axes.
A vertical lift axis carries gravity head-on. The moving mass, the carriage, and the payload all hang from the bearing arrangement, and gravity acts parallel to the shaft. The load is steady, it never reverses during normal cycling, and it grows whenever the payload grows. A wrist roll axis behaves the same way when the wrist flange points downward.
Tilted axes produce axial load from the same gravity vector. A joint at 30 degrees from horizontal splits its weight into a radial part and an axial part. The split follows the sine and cosine of the tilt angle. A 20 kg arm segment tilted 30 degrees puts about half of its weight force, roughly 98 N, into the axial direction and about 170 N into the radial direction.
| Load source | When it dominates | How to estimate it |
|---|---|---|
| Gravity on a vertical axis | Steady state, every cycle | F = m x g |
| Linear acceleration and braking | Every move profile, peak at turnaround | F = m x a |
| Payload change | Pick and place stations | F = change in mass x g |
| Emergency stop and collision | Rare, high magnitude | F = m x (g + stop deceleration) |
The last row matters more than its frequency suggests. A robot that handles a crash stop once a month still needs a bearing that survives that single event without permanent raceway damage.
02Turn Mass Into a Load Budget Before Opening the Catalog
Worked example, with stated assumptions. A vertical axis moves a carriage, a gripper, and a payload with a combined mass of 4 kg. Gravity alone produces 4 x 9.81 m/s2, about 39 N of steady axial load. The servo accelerates the axis at 10 m/s2, about 1 g, which adds 40 N during each move. Worst case during a normal cycle is roughly 79 N axial.
Compare that with the static rating of a small 6200-series part. The 6203 bearing carries a C0r of 4.78 kN, which is 4,780 N. The 79 N working load sits at about 2 percent of the static rating before any safety factor is applied. Even a design safety factor of 5 leaves the bearing far inside its static envelope. This is the normal result for robot axes: mass-driven axial loads are small against published static ratings. The interesting checks come later, when radial load is added and when the load cycles millions of times.
Load budget rule: convert every mass into force with F = m x g for steady load and F = m x a for acceleration, then add the two for the worst normal cycle. Keep the result in newtons next to the bearing rating in kilonewtons. Watch the units: 1 kN = 1,000 N.
03How Much Axial Load a Deep Groove Bearing Actually Carries
Yuanhe product pages state the rule directly: single-row deep groove geometry handles combined radial and bidirectional axial loads, and axial capacity runs at approximately 25 percent of the radial dynamic rating. Applied to the catalog, that gives a usable axial ceiling for each size.
| Model | Bore x OD x width | Cr dynamic | C0r static | Approx axial, 25 percent of Cr |
|---|---|---|---|---|
| 6203 | 17 x 40 x 12 mm | 9.58 kN | 4.78 kN | about 2.4 kN |
| 6204 | 20 x 47 x 14 mm | 12.8 kN | 6.55 kN | about 3.2 kN |
| 6205 | 25 x 52 x 15 mm | 14.0 kN | 7.85 kN | about 3.5 kN |
| 6206 | 30 x 62 x 16 mm | 19.5 kN | 11.3 kN | about 4.9 kN |
Why does the ceiling sit at 25 percent? When axial load arrives, the balls climb the raceway shoulder and the contact area shifts toward the edge of the groove. Raceway shoulders on a deep groove bearing are sized for radial dominance. They accept a substantial axial share, but they are not thrust surfaces. Stay under the 25 percent guideline and the contact ellipse stays inside the machined raceway.
Rings and balls on these sizes are GCr15 bearing steel, the AISI 52100 equivalent, through-hardened to HRC 60-64. Standard production runs to ABEC-1 and ABEC-3 tolerance classes with 100 percent dimensional inspection per batch, so the load path assumption in the rating holds part to part.
04Combined Loads Change the Calculation
Robot axes rarely load a bearing in a single direction. The same joint that carries axial gravity also carries the radial weight of the arm structure and the reaction of the motor drive. Rating a bearing on the axial number alone is the mistake this section exists to prevent.
ISO 281, the standard method for dynamic load rating, converts a combined load into a single equivalent radial load P. While the axial share stays small relative to the radial load, the equivalent load is simply the radial load Fr. Once the ratio Fa/Fr passes a boundary value, the calculation becomes P = 0.56 x Fr + Y x Fa. The factor 0.56 and the load factor Y come from the standard tables, and Y grows as the axial share grows relative to the static rating. Life then follows the cubic law: L10 scales with the cube of the load ratio. A 26 percent rise in equivalent load cuts calculated life roughly in half.
Two practical readings follow. First, an axial component that stays below the radial load is inexpensive: the 0.56 weighting actually reduces the radial term. Second, an axial component that approaches the radial load pulls Y into the calculation and the effective load climbs fast. The axial capacity check and the combined load check are two different gates. Run both.
05Peak Loads and Shock Are a Static Rating Problem
Dynamic rating Cr assumes rotating load and counts fatigue. A crash stop or a dropped payload is a different failure mode: permanent brinelling of the raceway at the ball contact points. Static rating C0r governs that mode. A single overload above the static envelope leaves a visible dent, and every later revolution runs over it.
Stay with the 4 kg example. A hard stop at 5 g deceleration produces 4 x 9.81 x 5, roughly 196 N axial for the duration of the stop. The 6204 bearing, a 20 mm bore part with C0r of 6.55 kN, takes that momentary 0.2 kN without measurable risk. The margin collapses only when the moving mass grows into the hundreds of kilograms or when the stop deceleration is built into the cycle profile as a regular event.
Bearing practice treats shock with a static safety factor. Common guidance puts the minimum at 1.5 for smooth loads and higher, in the 2 to 3 range, where shock or vibration is expected. Compare C0r against the worst momentary load times that factor, not the steady load.
06When Axial Demand Outgrows the Deep Groove Bearing
Three signals say the load sheet has outgrown a deep groove bearing. The axial share passes the 25 percent guideline. The joint spec includes a moment rating rather than a simple force. Or the axis needs measurable stiffness and controlled play at the contact, which a deep groove bearing cannot adjust once installed.
Moment load deserves the sharpest eye. A single deep groove bearing has no moment capacity to speak of. Designers solve this by mounting two bearings spaced apart on the shaft, which converts the applied moment into a radial force couple. That works when the housing has axial length to spare. Robot wrist and shoulder joints usually do not, and that is the point where a crossed roller bearing enters the picture: one compact unit carries combined radial, axial, and moment loads, with integrated rings and factory-set preload taking up the play that a deep groove pair leaves as tolerance stack.
The decision rule used on robot programs: deep groove bearing for axes where the force sheet lists forces, crossed roller bearing for joints where the sheet lists moments. For the general mechanics of axial capacity without the robot context, the earlier deep groove ball bearing axial load capacity explained post covers the same ground in catalog terms.
07Building the Axis Load Sheet for Production
A load sheet that survives review reads top to bottom: axis orientation, moving mass, gravity force, acceleration peak, worst combined radial and axial load, and the static check against C0r. Grease operating range belongs on the same sheet. Standard polyurea grease on these bearings runs -20°C to 120°C continuous, with high-temperature options to 150°C for OEM orders, which covers the thermal side of a servo-driven axis.
Send the finished load sheet, not just the bearing size, when you request a quote. Yuanhe checks the axial share, the combined load path, and the seal choice against the same published ratings used in this article, and flags the joint where the sheet points to a crossed roller bearing.
08Frequently Asked Questions
Can a deep groove ball bearing run on pure axial load?
Yes, within the published guideline. Yuanhe rates axial capacity at approximately 25 percent of the radial dynamic rating, and the geometry takes axial load in both directions. A design that lives under that ceiling with a safety factor is sound. A design that parks at the ceiling for the full duty cycle should move up a size or change bearing class.
How do I separate axial from radial load on a tilted axis?
Project the force vector onto the shaft. The component parallel to the shaft axis is axial, the component perpendicular to it is radial. For a mass hanging on an axis tilted at angle A from horizontal, the axial share is mass x g x sin(A) and the radial share is mass x g x cos(A). At 30 degrees the axial share is half the total weight.
When does a vertical axis need more than a deep groove bearing?
When the load sheet starts listing moment and stiffness instead of force. A deep groove bearing carries force; it cannot carry an applied moment alone and it has no built-in preload. Joints with large overhung payloads or tight backlash requirements move to a crossed roller bearing, which handles radial, axial, and moment load in one unit.
Does radial clearance CN or C3 change the axial rating?
No. Clearance changes how the bearing runs, not the published load ratings. CN is standard, and C3 suits motors where the inner ring runs 10°C to 20°C hotter than the outer ring in continuous duty. The axial ceiling stays the same in either class.
