Table of Contents
Rambo
Intro
Slewing bearings (turntable bearings) are critical components in heavy-duty rotating systems where axial load, radial load, and tilting moment act simultaneously. Unlike standard rolling bearings, slewing bearings are typically selected based on static load capacity, structural rigidity, and safety margin, rather than fatigue life alone.
This guide provides a step-by-step engineering method to calculate loads, evaluate safety factors, and select the correct slewing bearing for demanding applications such as cranes, excavators, wind turbines, and rotary platforms.
1. Load System Overview (Engineering Perspective)
A slewing bearing operates under a combined load system, which includes:
Axial Load (Fa)
- Acts parallel to the axis
- Usually the dominant load component
- Comes from structure weight, payload, or vertical force
Radial Load (Fr)
- Acts perpendicular to axis
- Typically smaller than axial load
- Influences raceway stress and rolling element distribution
Tilting Moment (M)
- Caused by eccentric load distribution
- Most critical factor in real applications
- Directly affects stability and bolt load
👉 In most real-world designs:
Moment load governs bearing size selection
2. Equivalent Load Calculation (Core Formula)
To simplify complex load combinations, engineers convert loads into an equivalent load:
P=X⋅Fr+Y⋅FaP = X \cdot F_r + Y \cdot F_a
Where:
- PP = equivalent load (N)
- FrF_r = radial load (N)
- FaF_a = axial load (N)
- X,YX, Y = load coefficients (provided by manufacturer)
Engineering Insight
- For slewing bearings:
- YY is typically 1.2 – 2.0
- XX is typically 0.4 – 0.7
👉 Meaning:
Axial load dominates equivalent load calculation
3. Tilting Moment Calculation (Critical Step)
Tilting moment determines whether the bearing will remain stable under offset loads:
M=F⋅LM = F \cdot L
Where:
- MM = tilting moment (Nm)
- FF = applied force (N)
- LL = load offset distance (m)
Engineering Insight
- Even small forces can create large moments if distance is large
- In cranes and excavators, moment load often exceeds axial influence
4. Static Load Safety Factor (Design Requirement)
Slewing bearings are selected using static safety factor (S):
S=C0PS = \frac{C_0}{P}Where:
- C0C_0 = static load rating
- PP = equivalent load
Recommended Safety Factors
| Application Type | Safety Factor |
|---|---|
|
Light duty |
≥ 1.3 |
|
Standard industrial |
≥ 1.5 |
|
Heavy-duty / shock |
≥ 2.0 |
|
Wind / critical systems |
≥ 2.5 |
Engineering Insight
- Shock loads and dynamic conditions require higher safety margins
- Undersizing leads to permanent deformation (Brinelling)
5. Combined Load & Moment Selection Method
In real applications, selection must consider:
✔ Load + Moment Diagram
Manufacturers provide diagrams showing allowable combinations of:
- Axial load
- Tilting moment
👉 The working point must fall within the safe zone
✔ Bolt Load Distribution
- Moment load increases stress on mounting bolts
- Uneven preload can cause localized failure
✔ Structural Rigidity
- Bearing performance depends on:
- Housing stiffness
- Mounting flatness
👉 Weak structure = bearing failure (even if calculation is correct)
6. Full Example Calculation (Engineering Case)
📌 Application:
Hydraulic excavator upper structure
🔹 Given Data:
- Axial load Fa=150,000F_a = 150,000 N
- Radial load Fr=40,000F_r = 40,000 N
- Load offset L=1.5L = 1.5 m
🔹 Step 1: Calculate Tilting Moment
M=150,000×1.5=225,000 NmM = 150,000 \times 1.5 = 225,000 \, Nm
🔹 Step 2: Equivalent Load
Assume:
- X=0.5X = 0.5
- Y=1.6Y = 1.6
P=0.5×40,000+1.6×150,000P = 0.5 \times 40,000 + 1.6 \times 150,000 P=20,000+240,000=260,000 NP = 20,000 + 240,000 = 260,000 \, N
🔹 Step 3: Select Bearing
Assume candidate bearing:
- Static rating C0=550,000C_0 = 550,000 N
S=550,000260,000≈2.12S = \frac{550,000}{260,000} \approx 2.12
🔹 Step 4: Evaluation
✔ Safety factor > 2.0 → suitable for heavy-duty
✔ Moment within allowable range → acceptable
👉 Final selection: Double-row ball or three-row roller slewing bearing
7. Bearing Type Selection Logic
| Type | Advantage | Application |
|---|---|---|
|
Four-point contact ball |
Cost-effective |
General machinery |
|
Double-row ball |
Higher axial capacity |
Excavators |
|
Crossed roller |
High precision |
Robotics |
|
Three-row roller |
Maximum load |
Mining cranes |
8. Material & Heat Treatment
Common materials:
- 42CrMo → high strength
- 50Mn → cost-effective
Heat treatment:
- Raceway induction hardening
- Gear surface hardening
👉 Directly affects:
- Wear resistance
- Load capacity
9. Equivalent & Replacement
FHD slewing bearings are interchangeable with:
- SKF
- Schaeffler
Cross-reference and reverse engineering support available.
10. Common Engineering Mistakes
❌ Ignoring tilting moment
→ Most common failure cause
❌ Using dynamic rating instead of static
→ Wrong selection basis
❌ Insufficient bolt preload
→ Leads to uneven load distribution
❌ Poor mounting surface
→ Causes deformation and premature failure
11. FAQ
Q1: Why is slewing bearing selection based on static load?
Because slewing bearings operate at low speeds, failure is usually due to deformation rather than fatigue.
Q2: What is the most critical parameter?
Tilting moment, especially in offset load conditions.
Q3: Can I directly replace SKF slewing bearings?
Yes, with proper cross-reference and engineering validation.
Q4: What safety factor should I use?
- Standard: ≥1.5
- Heavy-duty: ≥2.0
Q5: Do you provide engineering support?
Yes, FHD Bearings offers full calculation and selection assistance.
