Appearance
A hydropower turbine manufacturer was producing 120 MW Francis turbine main shafts in 42CrMo steel. Each shaft — 5,500 mm long with a 200 mm central bore — required a through-bore for oil supply and condition monitoring access. The existing process used conventional twist drilling from both ends with a meeting accuracy of ±3 mm at the junction, requiring manual weld repair and re-machining on 20 % of shafts. By installing a dedicated BTA drilling system with counter-rotation capability, the bore was completed in a single pass from one end with straightness of 0.02 mm per metre. Cycle time dropped from 24 hours (including repair time) to 4 hours per shaft, and the weld repair rate was eliminated entirely. The €1.6 million investment was recovered in 22 months through reduced cycle time and repair cost.
Hydropower Components Requiring Deep Hole Drilling
Hydropower generation equipment includes some of the largest rotating machinery in the world, with components that require deep hole drilling for oil passages, condition monitoring access, assembly features, and structural weight reduction.
Turbine main shafts: The main shaft connects the turbine runner to the generator rotor. For Francis and Pelton turbines, these shafts typically range from 3,000–12,000 mm in length with diameters of 400–1,500 mm. A central through-bore (100–400 mm diameter) is drilled the full length of the shaft to provide an oil passage for the guide bearing, a conduit for instrumentation cables, and access for shaft straightness inspection.
Runner drain holes: Francis turbine runners contain drain holes on the upper crown (typically 20–50 mm diameter, 200–800 mm deep) to relieve pressure behind the runner and control axial thrust. These holes connect the crown cavity to the draft tube and must be positioned precisely between the blades.
Wicket gate servo cylinders: The wicket gate operating mechanism uses hydraulic servo-cylinders that require precision-drilled bores (40–150 mm diameter, 500–3,000 mm stroke) for reliable gate positioning under high forces.
Coupling bolt bores: Turbine shaft flanges are connected by multiple coupling bolts arranged in a bolt circle. The bolt holes (20–80 mm diameter, 200–800 mm depth through the flange) require precise positioning and straightness to ensure proper load distribution across the coupling.
Guide bearing housings: The turbine guide bearing housing contains a central bore and multiple oil passages drilled at angles to feed the bearing journals. These passages are typically 10–40 mm diameter with depth-to-diameter ratios of 10:1 to 30:1.
Head cover and bottom ring bores: These large Annbauteile contain multiple vertical and radial bores for stay vanes, wicket gate bushings, and seal housings. Bore diameters range from 10–120 mm with moderate depth.
Penstock and manifold penetration bores: Penstock branch pipes and manifold blocks require large-diameter through-bores at compound angles for flow passage connections. These are typically BTA or trepanned depending on diameter.
Turbine Main Shaft Central Bore
The central through-bore in the turbine main shaft is the most demanding deep hole drilling operation in hydropower component manufacturing.
Shaft bore functions:
- Oil supply passage to the upper and lower guide bearings
- Conduit for vibration monitoring probes and temperature sensors
- Access for shaft straightness inspection
- Hydraulic oil passage for Kaplan blade pitch control (Kaplan turbines)
Shaft dimensions by turbine type:
| Turbine type | Shaft length (mm) | Shaft OD (mm) | Bore diameter (mm) | Typical power (MW) |
|---|---|---|---|---|
| Small Francis | 3,000–5,000 | 400–700 | 80–150 | 10–50 |
| Medium Francis | 5,000–8,000 | 700–1,200 | 150–250 | 50–200 |
| Large Francis | 8,000–12,000 | 1,200–1,500 | 250–400 | 200–800 |
| Pelton (horizontal) | 4,000–8,000 | 500–900 | 100–200 | 50–300 |
| Kaplan | 5,000–10,000 | 600–1,200 | 200–350 | 30–200 |
| Pumped storage | 5,000–8,000 | 800–1,200 | 150–250 | 200–400 |
BTA drilling for main shafts:
BTA drilling is the standard method for turbine main shaft central bores. The process typically uses a single-pass drilling approach with the shaft rotating while the BTA head advances on a stationary tool carriage.
Key parameters for a 200 mm × 5,500 mm bore in 42CrMo steel:
- BTA head diameter: 200 mm (final bore, single pass)
- Cutting speed: 55–75 m/min
- Feed rate: 0.12–0.18 mm/rev
- Spindle speed (workpiece rotation): 90–120 rpm
- Coolant pressure: 12–20 bar
- Coolant flow: 600–900 L/min
- Estimated drilling time: 3.5–5 hours
Counter-rotation benefits:
For long shafts with high L/D ratios, counter-rotation (workpiece and tool rotating in opposite directions) provides:
- Improved straightness by cancelling the effects of shaft runout
- Reduced bore wall waviness by distributing tool deflection forces
- Better concentricity when the bore must align with the shaft OD centreline
Runner Drain Hole Drilling
Francis turbine runners require precisely positioned drain holes to equalise pressure between the crown cavity and the draft tube. These holes are drilled through the runner crown between the blade passages.
Drain hole specifications:
| Runner size | Crown thickness at drilling location | Drain hole diameter | Number of holes | Drilling method |
|---|---|---|---|---|
| Small (< 2 m diameter) | 30–60 mm | 15–25 mm | 4–8 | Gun drilling |
| Medium (2–5 m diameter) | 60–120 mm | 25–40 mm | 6–12 | Gun drilling |
| Large (> 5 m diameter) | 120–200 mm | 40–60 mm | 8–16 | BTA drilling |
Drilling challenges for runner drain holes:
The runner crown is a complex curved surface with limited access between blade passages. Drain holes must be drilled at compound angles that align with the blade geometry. Key challenges include:
- Drill entry on a curved surface — requires a spot-face or starter drill to prevent the drill from walking
- Limited space between blades for drill head access
- Varying material thickness along the hole path
- Stainless steel cladding on the flow surfaces (harder than base material)
Gun drilling parameters for runner drain holes (martensitic stainless steel crown, 250–350 HB):
| Hole diameter | Cutting speed (m/min) | Feed (mm/rev) | Coolant pressure (bar) |
|---|---|---|---|
| 20 mm | 25–40 | 0.03–0.06 | 60–90 |
| 30 mm | 22–35 | 0.04–0.07 | 50–80 |
| 40 mm | 20–30 | 0.04–0.08 | 45–70 |
Tip: For Francis turbine runner drain holes, drill from the band (draft tube side) toward the crown rather than from the crown downward. Drilling in this direction means the drill is pushing into decreasing material thickness, improving chip evacuation and reducing the risk of drill breakage. It also produces a cleaner exit hole on the crown surface, which is the visible surface in the assembled turbine.
Wicket Gate Servo Cylinders
The wicket gate operating mechanism uses high-pressure hydraulic cylinders (typically 100–200 bar) to position the guide vanes. These cylinders are mounted on the head cover and connected to the wicket gate operating ring.
Servo cylinder specifications:
| Turbine size | Cylinder bore (mm) | Stroke (mm) | Number per turbine | Operating pressure (bar) |
|---|---|---|---|---|
| Small | 50–80 | 300–500 | 2–4 | 100–140 |
| Medium | 80–140 | 500–800 | 4–8 | 120–160 |
| Large | 140–250 | 800–1,500 | 8–24 | 140–200 |
These cylinders are typically manufactured from seamless steel tube (ST52, 25CrMo4, or 42CrMo4) and require BTA drilling followed by skiving and burnishing. The drilling process is similar to other hydraulic cylinder manufacturing but with the added requirement of long-term corrosion resistance in water-filled turbine environments.
Coupling Bolt Bores
Turbine shaft flanges are massive components that transfer torque through precision-machined coupling bolts. The bolt bores require machining that is distinct from the main shaft central bore.
Bolt bore requirements:
- Diameter: 20–80 mm (depending on turbine size and torque)
- Length: 200–800 mm (through the flange pair or blind into the shaft)
- Tolerance: H7 for reamed bolt fit
- Position tolerance: ±0.1 mm on bolt circle diameter
- Surface finish: Ra < 1.6 µm for reamed bores
Manufacturing methods:
For through-bores in the flange, drilling is performed radially at the bolt circle radius. On large shafts, this is typically done on a horizontal boring mill with the shaft positioned vertically or horizontally. For blind bores (used in some designs where bolts thread into the shaft), gun drilling provides the required straightness and surface finish.
A Chinese patent (CN104325173A) describes a field machining method for large water turbine shaft coupling blind hole boring where the boring bar is supported by a thrust bearing on the shaft face and a guide bearing in the bore, enabling in-situ machining without shaft removal.
Materials for Hydropower Components
Shaft materials:
| Material | Yield strength (MPa) | Tensile strength (MPa) | Application | Drillability |
|---|---|---|---|---|
| 42CrMo (AISI 4140) | 650–950 | 800–1,200 | Francis and Pelton shafts, medium-large | Good |
| 34CrNiMo (AISI 4340) | 750–1,080 | 900–1,300 | Large shafts, high-stress | Moderate |
| 40CrNiMo | 700–1,000 | 850–1,200 | Pumped storage shafts | Moderate |
| S355 (AISI 1026) | 355 | 500–700 | Small shafts, low-head turbines | Excellent |
| 18Mn18N | 450–550 | 700–850 | Generator retaining rings | Moderate |
Runner materials:
| Material | Tensile strength (MPa) | Application | Drillability |
|---|---|---|---|
| GX4CrNi13-4 (13-4 martensitic SS) | 750–900 | Francis runners, pump turbines | Moderate |
| GX3CrNiMo13-6 | 800–950 | High-head runners | Moderate |
| 0Cr13Ni5Mo | 800–900 | Large runner castings | Moderate |
| S355J2 + stainless cladding | 500–700 | Kaplan runner hub | Good (varies with cladding) |
Material selection considerations:
- 42CrMo is the standard shaft material for medium-to-large hydro turbines. Its through-hardening capability and good machinability make it suitable for long BTA-drilled shafts.
- For the largest shafts (300+ MW), 34CrNiMo offers superior strength but requires reduced cutting speeds and more frequent tool changes in BTA drilling.
- Runner steels are typically 13-4 martensitic stainless steel castings with hardness of 250–350 HB. The high chromium content (13 %) causes abrasive wear on cutting tools.
BTA Drilling Parameters for Turbine Shafts
42CrMo / 34CrNiMo (normalised or Q&T, 200–320 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | RPM | Coolant pressure (bar) | Coolant flow (L/min) |
|---|---|---|---|---|---|
| 80 mm | 50–80 | 0.12–0.20 | 200–320 | 15–25 | 350–550 |
| 120 mm | 45–70 | 0.14–0.22 | 120–185 | 12–20 | 500–750 |
| 160 mm | 40–65 | 0.14–0.24 | 80–130 | 10–18 | 600–900 |
| 200 mm | 35–60 | 0.15–0.25 | 55–95 | 8–15 | 700–1,100 |
| 250 mm | 30–55 | 0.16–0.26 | 40–70 | 7–12 | 850–1,300 |
| 300 mm | 28–50 | 0.16–0.28 | 30–55 | 6–10 | 1,000–1,500 |
| 400 mm | 25–45 | 0.18–0.28 | 20–35 | 5–8 | 1,200–1,800 |
Cycle time estimation for shaft boring:
For a 200 mm × 5,500 mm shaft bore in 42CrMo steel:
- BTA feed: 0.18 mm/rev at 75 rpm = 13.5 mm/min
- Drilling time (5,500 mm): 407 min (6.8 hours)
- Setup time: 2 hours
- Retraction and tool change: 1 hour
- Total cycle: approximately 10 hours
For comparison, conventional twist drilling from both ends on a lathe requires 20–30 hours for the same bore.
Surface finish and accuracy expectations:
| Condition | Surface finish Ra | Diameter tolerance |
|---|---|---|
| As BTA drilled | 3.2–6.3 µm | IT9–IT10 |
| After rough boring | 1.6–3.2 µm | IT8–IT9 |
| After fine boring | 0.8–1.6 µm | IT7–IT8 |
| After roller burnishing | 0.2–0.8 µm | IT8–IT9 |
Warning: For turbine main shaft bores above 250 mm diameter, the volume of steel chips generated during BTA drilling exceeds 150 kg per metre of bore. The chip conveyor system must be sized for at least 500 kg/hr capacity, and the coolant filtration system must handle the corresponding chip load. A magnetic drum separator rated for 500 L/min minimum flow with a secondary paper filter is recommended. Without adequate chip handling, the BTA head can become buried in its own chips, causing chip packing that leads to guide pad seizure and catastrophic tool failure.
Gun Drilling for Small Hydropower Components
Smaller hydropower components — instrumentation ports, oil passages, drain holes — are gun drilled.
Gun drilling parameters for martensitic stainless steel (13-4, 250–350 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | RPM | Coolant pressure (bar) |
|---|---|---|---|---|
| 10 mm | 20–35 | 0.015–0.030 | 640–1,100 | 80–120 |
| 16 mm | 22–38 | 0.020–0.035 | 440–760 | 70–100 |
| 25 mm | 25–40 | 0.025–0.040 | 320–510 | 60–90 |
| 40 mm | 22–35 | 0.030–0.050 | 175–280 | 50–80 |
For 42CrMo shaft material:
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | Coolant pressure (bar) |
|---|---|---|---|
| 16 mm | 40–65 | 0.04–0.08 | 40–70 |
| 25 mm | 35–60 | 0.05–0.10 | 35–60 |
| 40 mm | 30–55 | 0.06–0.12 | 30–50 |
Quality Requirements
Hydropower components are subject to stringent quality standards driven by the long service life (50+ years), high reliability requirements, and safety criticality of rotating machinery.
Bore quality specifications:
| Component | Diameter tolerance | Straightness | Surface finish Ra | Inspection method |
|---|---|---|---|---|
| Main shaft central bore | H9–H10 | < 0.03 mm/m | < 3.2 µm | Ultrasonic, air gauging |
| Runner drain hole | ±0.2 mm | < 0.5 mm | < 6.3 µm | Go/no-go gauge |
| Coupling bolt bore (reamed) | H7 | < 0.02 mm per 100 mm | < 1.6 µm | CMM, plug gauge |
| Servo cylinder bore | H8–H9 | < 0.05 mm/m | < 0.4 µm (after SRB) | Air gauging |
| Guide bearing bore | H7 | < 0.02 mm | < 0.8 µm | CMM |
Shaft straightness verification:
After the central bore is completed, the shaft straightness must be verified. This is typically done using:
- A laser alignment system aligned to the bore centreline
- Dial indicator measurements at the shaft OD at multiple positions
- Ultrasonic wall thickness measurement to verify concentricity of bore to OD
For large shafts, straightness within 0.03 mm per metre of length is the standard acceptance criterion.
Nondestructive testing:
- Ultrasonic testing: Full volumetric inspection of the shaft after drilling
- Magnetic particle inspection: Surface and near-surface defect detection
- Borescope inspection: Visual examination of the full bore length
- Hardness testing: Verification of material condition after heat treatment
Machine Configurations for Hydropower Components
Large horizontal BTA machines:
For turbine shaft bores, machines with 10–15 metre bed length, 75–150 kW spindle power, and 1,500+ L/min coolant capacity are required. Key manufacturers include Shin-Il, TBT, UNISIG, and Precihole.
Workpiece rotation systems:
Turbine shafts are typically rotated by a headstock and tailstock system, with the shaft supported on adjustable steady rests. The BTA tool carriage advances along a separate way parallel to the shaft axis. The ratio of shaft rotation speed to tool feed determines the cutting speed and bore finish.
Vertical BTA machines for large runners:
For runner drain hole drilling, vertical or gantry-type machines with a rotary table and tilting drilling head are used. The runner is positioned on the table and indexed to each drain hole position. The drilling head can be angled to match the hole entry angle.
On-site boring equipment:
For field maintenance of large hydro turbines, portable boring systems are used for in-situ coupling bolt bore drilling and flange facing. These systems clamp to the shaft flange and provide guided drilling without shaft removal.
Troubleshooting Hydropower Component Drilling
| Symptom | Likely cause | Correction |
|---|---|---|
| Shaft bore straightness > 0.05 mm/m | Shaft residual stress release during drilling | Stress relieve shaft before final BTA pass; increase steady rest support |
| Runner drain hole position misses between blades | Curved entry surface causes drill walk | Cut spot face at entry before drilling; use stub starter drill |
| Coupling bolt hole misalignment on bolt circle | Shaft thermal growth during long drilling cycle | Allow shaft to stabilise before final reaming; check coolant temperature |
| Guide pad seizure in large-diameter BTA head | Chip packing from inadequate coolant flow | Increase coolant flow by 20 %; verify chip tube is clear |
| Oversized bore at shaft entry end | Worn guide pads on BTA head entry side | Replace guide pads; verify head diameter with micrometer |
| Servo cylinder bore surface pitting | Chloride pitting from coolant residues | Improve post-drilling cleaning; use corrosion-inhibited coolant |
| Low material removal rate in 13-4 SS runner | Work hardening at cutting edge | Reduce cutting speed to 20–25 m/min; increase feed; use AlTiN-coated carbide |
| Chatter marks in long shaft bore | Tool resonance at critical speed | Adjust RPM to avoid resonant frequency; increase tool stiffness |
| BTA chip tube blockage in 200 mm shaft bore | Oversized chips bridging tube ID | Check chip breaker geometry; reduce feed by 10–15 % to shorten chip length |
Frequently Asked Questions
What is the most common deep hole drilling method for hydropower turbine main shafts? BTA drilling is the standard method for turbine shaft central bores (80–400 mm diameter, up to 12,000 mm length). It provides superior straightness, surface finish, and material removal rate compared to conventional twist drilling from both ends.
What materials are used for hydropower turbine shafts? 42CrMo (AISI 4140) is the most common material for medium-to-large shafts. 34CrNiMo (AISI 4340) is used for the largest high-stress shafts. Small and low-head turbines may use S355 steel.
How is straightness controlled in long shaft BTA boring? Counter-rotation (workpiece and tool rotating in opposite directions), proper steady rest positioning, stress relief heat treatment before final boring, and stabilised coolant temperature are the primary methods for achieving straightness within 0.03 mm per metre.
What coolant pressure is required for BTA drilling turbine shafts? 5–25 bar depending on bore diameter. Large bores (250–400 mm) require 5–12 bar with flow rates up to 1,800 L/min. Smaller bores (80–150 mm) require 15–25 bar.
How are Francis turbine runner drain holes drilled? Runner drain holes (20–60 mm diameter through the crown) are gun drilled or BTA drilled depending on size. The runner is positioned on a rotary table with indexed positions for each drain hole. Drilling from the band toward the crown provides better chip evacuation.
What is the typical cycle time for BTA drilling a turbine main shaft? For a 200 mm × 5,500 mm shaft bore in 42CrMo, the BTA drilling cycle is approximately 7 hours. Including setup and retraction, total cycle time is approximately 10 hours — compared to 20–30 hours for conventional methods.
What quality inspections are required for turbine shaft bores? Ultrasonic testing (full volumetric), air gauging (diameter at multiple depths), laser alignment (straightness), borescope (surface condition), and magnetic particle inspection (end faces).
How does hydropower shaft drilling differ from other large shaft drilling (marine, wind)? Hydropower shafts have lower L/D ratios than marine shafts but larger diameters. The emphasis is on bore straightness for bearing oil supply reliability and instrumentation access, rather than the extreme surface finish required for seal compatibility in hydraulic cylinders.
Can turbine shaft bores be drilled on a conventional lathe? Yes, for smaller shafts (< 5,000 mm length, < 100 mm bore), the central bore can be drilled on a heavy-duty lathe using twist drills or a BTA attachment with through-coolant adaptor. For larger shafts, dedicated BTA machines are required.
What is the trend in hydropower turbine shaft design for deep hole drilling? The trend is toward larger single-unit capacities (500–800 MW for pumped storage) requiring larger shaft diameters and longer bores. This drives demand for BTA machines with higher torque (150+ kW), longer beds (15+ metres), and higher coolant capacity (2,000+ L/min). Condition monitoring integration through the central bore is also becoming standard, requiring cleaner bores and integral conduit routing features.
Summary
| Aspect | Turbine main shaft bore | Runner drain holes | Coupling bolt bores | Servo cylinder bore |
|---|---|---|---|---|
| Typical bore diameter | 80–400 mm | 15–60 mm | 20–80 mm | 50–250 mm |
| Typical length/depth | 3,000–12,000 mm | 30–200 mm (crown thickness) | 200–800 mm | 300–1,500 mm |
| Drilling method | BTA | Gun drilling / BTA | Gun drilling / boring | BTA + SRB |
| Typical material | 42CrMo, 34CrNiMo | 13-4 martensitic SS | 42CrMo | 25CrMo4, 42CrMo4 |
| Cutting speed | 25–80 m/min | 20–40 m/min | 30–60 m/min | 35–80 m/min |
| Feed | 0.12–0.28 mm/rev | 0.03–0.08 mm/rev | 0.06–0.15 mm/rev | 0.10–0.24 mm/rev |
| Coolant pressure | 5–25 bar | 45–90 bar | 30–70 bar | 8–35 bar |
| Straightness requirement | < 0.03 mm/m | < 0.5 mm | < 0.02 mm per 100 mm | < 0.05 mm/m |
| Surface finish Ra | < 3.2 µm | < 6.3 µm | < 1.6 µm (reamed) | < 0.4 µm (SRB) |
Deep hole drilling for hydropower generation components is dominated by large-scale BTA boring of turbine main shafts, supported by gun drilling for runner drain holes, coupling bolt bores, and instrumentation passages. The combination of large diameters, extreme lengths, high-strength alloy steels, and stringent straightness requirements makes hydropower component drilling one of the most demanding applications in the deep hole drilling industry. As the global push for renewable energy drives investment in new hydropower and pumped storage projects — particularly in Asia, South America, and Europe — the demand for large-capacity BTA drilling machines and specialised shaft boring expertise will continue to grow.