Appearance
A European elevator manufacturer was producing 8,000 hydraulic elevator ram cylinders per year for passenger lifts in low-to-medium rise buildings. Each cylinder — 120 mm bore × 4,500 mm stroke in C45 steel — required a precision bore for the plunger seal interface. The existing honing process after conventional boring required 45 minutes per cylinder and produced inconsistent surface finish (Ra 0.4–0.8 µm). By replacing the boring and honing operations with BTA drilling followed by single-pass skiving and roller burnishing, cycle time dropped to 18 minutes per cylinder, surface finish was consistently Ra 0.15–0.25 µm, and seal leakage warranty claims fell by 70 %. The investment in a combined BTA and SRB machining centre was recovered in 20 months through reduced cycle time, elimination of honing stones and abrasives, and lower warranty costs.
Lifting Equipment Components Requiring Deep Hole Drilling
Elevator and lifting equipment hydraulic cylinders operate at lower pressures than heavy equipment cylinders but require extremely long strokes, high surface finish for seal longevity, and corrosion resistance for long service life.
Hydraulic elevator ram cylinders: These are single-acting cylinders that lift the elevator car directly or through a roping system. The cylinder barrel contains the plunger (ram) and is typically mounted vertically in a casing. Bore diameters range from 50–290 mm with stroke lengths of 2,000–6,000 mm for passenger elevators and up to 30,000 mm for heavy-duty freight elevators.
Plunger (ram) tubes: The plunger itself is a hollow or solid steel tube that extends from the cylinder to lift the car. Plunger tubes require a smooth outer surface finish (Ra < 0.3 µm) for seal compatibility and are often hard chrome plated. The inner bore, if present, must be clean and free of burrs.
Scissor lift cylinders: Scissor lift platforms for warehousing and industrial use contain multiple hydraulic cylinders that raise and lower the platform. These cylinders have shorter strokes (500–2,000 mm) with smaller bores (30–80 mm) and are typically double-acting.
Dock leveller cylinders: Loading dock levellers use hydraulic cylinders to raise and lower the leveller platform. These cylinders have moderate strokes (400–1,500 mm) and bores (40–100 mm) with heavy-duty seal requirements.
Vehicle lift cylinders: Car lift and truck lift cylinders used in service stations and workshops require bores of 60–150 mm with strokes of 1,500–2,500 mm. These operate at higher pressures (150–250 bar) than elevator cylinders.
Platform lift and wheelchair lift cylinders: Accessibility lifts for public buildings and private residences use compact cylinders with bores of 20–60 mm and strokes of 500–1,500 mm.
Cylinder Sizing by Capacity
| Lift type | Capacity (kg) | Cylinder bore (mm) | Stroke (mm) | Working pressure (bar) |
|---|---|---|---|---|
| Passenger elevator (low-rise) | 320–1,000 | 50–120 | 2,000–4,500 | 30–70 |
| Passenger elevator (medium-rise) | 1,000–2,500 | 120–200 | 4,500–8,000 | 40–80 |
| Freight elevator | 2,500–10,000 | 150–290 | 6,000–30,000 | 50–100 |
| Scissor lift (industrial) | 500–5,000 | 40–100 | 1,000–3,000 | 100–200 |
| Vehicle lift (car) | 3,000–5,000 | 80–150 | 1,500–2,500 | 150–250 |
| Dock leveller | 6,000–15,000 | 60–120 | 400–1,500 | 80–150 |
| Wheelchair platform lift | 250–500 | 25–50 | 500–1,500 | 40–80 |
Materials for Lifting Equipment Cylinders
Cylinder barrel materials:
| Material | Tensile strength (MPa) | Application | Weldability | Drillability |
|---|---|---|---|---|
| C45 (AISI 1045) | 600–800 | Standard elevator cylinders | Good | Excellent |
| 25CrMo4 (AISI 4130) | 700–900 | Premium lifts, higher pressure | Good | Good |
| 42CrMo4 (AISI 4140) | 900–1,200 | Heavy-duty freight lifts | Fair | Good |
| ST52 (AISI 1026) | 500–700 | Light-duty platform lifts | Excellent | Excellent |
| E355 (BS 6323) | 590–770 | Seamless cylinder tubes | Good | Excellent |
Plunger materials:
| Material | Tensile strength (MPa) | Surface finish | Corrosion resistance |
|---|---|---|---|
| C45 hard chrome plated | 600–800 | Ra < 0.3 µm (plated) | Good (with chrome) |
| 25CrMo4 induction hardened | 800–950 | Ra < 0.2 µm (ground) | Good |
| 17-4PH stainless | 1,100 | Ra < 0.4 µm (passivated) | Excellent (no plating) |
| Nitrided 42CrMo4 | 1,000–1,200 | Ra < 0.2 µm (ground) | Good |
Material selection considerations:
- C45 is the most common barrel material for standard elevator cylinders due to low cost, good machinability, and adequate strength for the low operating pressures (30–80 bar).
- 25CrMo4 offers higher strength for freight elevators and long-stroke cylinders where column buckling of the barrel is a concern.
- Chrome-plated plunger rods are standard for elevator applications. Hard chrome thickness of 25–50 µm with a surface finish of Ra < 0.3 µm provides the wear resistance and low friction required for millions of operating cycles.
- For plunger tubes, seamless cold-drawn precision steel tubes (SK5 quality per DIN 2391) are often used, requiring only internal bore drilling and external finishing.
BTA Drilling Parameters for Lift Cylinders
BTA drilling is the primary method for elevator and lifting cylinder bores due to the combination of large diameter and long stroke length.
C45 / E355 steel (normalised, 160–210 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | RPM | Coolant pressure (bar) | Coolant flow (L/min) |
|---|---|---|---|---|---|
| 40 mm | 60–90 | 0.10–0.18 | 480–720 | 25–40 | 150–250 |
| 50 mm | 55–85 | 0.12–0.20 | 350–540 | 22–35 | 200–320 |
| 60 mm | 50–80 | 0.12–0.22 | 260–420 | 18–30 | 250–400 |
| 80 mm | 45–70 | 0.14–0.24 | 180–280 | 15–25 | 350–550 |
| 100 mm | 40–65 | 0.16–0.26 | 125–210 | 12–22 | 450–700 |
| 120 mm | 35–60 | 0.16–0.28 | 95–160 | 10–18 | 500–800 |
| 160 mm | 30–50 | 0.18–0.28 | 60–100 | 8–15 | 650–1,000 |
| 200 mm | 25–45 | 0.18–0.30 | 40–70 | 7–12 | 800–1,200 |
25CrMo4 / 42CrMo4 (Q&T, 220–310 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | Coolant pressure (bar) |
|---|---|---|---|
| 60 mm | 40–65 | 0.10–0.18 | 20–35 |
| 100 mm | 35–55 | 0.12–0.20 | 15–28 |
| 120 mm | 30–50 | 0.14–0.22 | 12–22 |
| 160 mm | 25–45 | 0.14–0.24 | 10–18 |
Cycle time estimation for elevator cylinders:
For a 120 mm × 4,500 mm elevator cylinder barrel in C45 steel:
- BTA drilling feed: 0.18 mm/rev at 120 rpm = 21.6 mm/min
- Drilling time (4,500 mm): 208 minutes (3.5 hours)
- Retraction time: 8 minutes
- Total drilling cycle per barrel: approximately 3.6 hours
With a 2-spindle BTA machine, effective cycle time per barrel is 1.8 hours.
Gun Drilling for Small Lifting Components
Smaller lifting components — platform lift cylinders, scissor lift pivot bores, and accessory cylinder bores under 40 mm — are typically gun drilled.
Gun drilling parameters (C45, ST52 steels):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | RPM | Coolant pressure (bar) |
|---|---|---|---|---|
| 16 mm | 60–100 | 0.04–0.08 | 1,200–2,000 | 40–70 |
| 20 mm | 55–90 | 0.05–0.09 | 880–1,400 | 35–65 |
| 25 mm | 50–85 | 0.05–0.10 | 640–1,080 | 30–60 |
| 32 mm | 45–80 | 0.06–0.12 | 450–800 | 30–55 |
| 40 mm | 40–75 | 0.06–0.12 | 320–600 | 25–50 |
Skiving and Burnishing for Lift Cylinders
Skiving and roller burnishing (SRB) is the standard finishing method for elevator and lifting cylinders after BTA drilling.
SRB benefits for lift cylinders:
- Single-pass finishing eliminates separate honing operation
- Achieved surface finish: Ra 0.1–0.4 µm (sufficient for hydraulic elevator seals)
- Surface work hardening: 10–20 % increase improves wear resistance
- Dimensional accuracy: H8–H9 diameter tolerance
- Cylindricity: < 0.02 mm per 1,000 mm
- Cycle time: 5–15 minutes for a 4,500 mm elevator cylinder
SRB parameters:
| Bore diameter | Skiving stock (mm) | Burnishing stock (mm) | Feed (mm/min) | Spindle speed (RPM) |
|---|---|---|---|---|
| 40–60 mm | 0.12–0.20 | 0.03–0.05 | 400–800 | 400–600 |
| 80–120 mm | 0.15–0.25 | 0.03–0.05 | 300–600 | 250–450 |
| 140–200 mm | 0.20–0.30 | 0.04–0.06 | 200–400 | 150–300 |
| 200–290 mm | 0.25–0.35 | 0.05–0.07 | 150–300 | 100–200 |
Surface finish progression:
| Condition | Ra value | Application |
|---|---|---|
| As BTA drilled | 3.2–6.3 µm | Before SRB |
| After skiving | 0.8–1.6 µm | Intermediate |
| After burnishing | 0.1–0.4 µm | Final — seal surface ready |
| After optional wiper pass | 0.05–0.15 µm | Premium lifts |
Chrome Plating of Plungers
For elevator plungers (rams), hard chrome plating of the outer diameter is standard. This is not a deep hole drilling operation but directly affects the bore requirements.
Plating specifications for elevator plungers:
| Parameter | Standard | Premium |
|---|---|---|
| Chrome thickness | 25–40 µm | 40–60 µm |
| Hardness | 850–950 HV | 900–1,000 HV |
| Surface finish after plating | Ra < 0.4 µm | Ra < 0.2 µm |
| Porosity | < 3 spots per dm² | < 1 spot per dm² |
| Corrosion resistance (salt spray) | 200 hours min | 500 hours min |
Tip: For elevator cylinder plungers operating in outdoor or marine environments (dock levellers, external platform lifts), specify dual-layer chrome plating — a 5–10 µm microporous chrome layer over the standard hard chrome layer. The microporous layer distributes corrosion microcells across thousands of pores rather than concentrating them at a few defects, extending corrosion resistance by 3–5× compared to standard hard chrome.
Production Line Configuration
Elevator cylinder barrel production follows a linear process flow optimised for long-stroke components.
Typical production steps for elevator cylinders:
- Tube or bar cutting: Material cut to length with ±2 mm tolerance for stroke allowance
- Rough external turning: OD turned for mounting threads and port bosses
- Deep hole drilling: BTA drilling for the main bore (≥ 40 mm) or gun drilling (< 40 mm)
- SRB finishing: Single-pass skiving and roller burnishing
- Inspection: Air gauging of bore diameter; surface finish measurement
- End preparation: Threading or grooving for end cap and gland
- Port drilling and tapping: Radial holes for hydraulic ports
- Welding: Base cap and clevis attachment welding (if applicable)
- Hydrostatic testing: 1.5× working pressure
- Final cleaning and assembly
Production rates for elevator cylinders:
| Cylinder type | BTA + SRB cycle | Daily output (16 hr, 2-spindle) |
|---|---|---|
| Passenger lift (120 mm × 4,500 mm) | ~2 hr | 6–8 cylinders |
| Freight lift (200 mm × 8,000 mm) | ~4 hr | 3–4 cylinders |
| Scissor lift (60 mm × 1,500 mm) | ~35 min | 25–30 cylinders |
| Platform lift (40 mm × 1,000 mm) | ~20 min | 40–50 cylinders |
Quality Requirements
Elevator and lifting cylinders must meet specific quality standards for reliable operation and safety certification.
Bore quality specifications:
| Parameter | Passenger elevator | Freight elevator | Scissor lift |
|---|---|---|---|
| Diameter tolerance | H9 (±0.054 mm for 100 mm) | H8–H9 | H9–H10 |
| Surface finish Ra | < 0.4 µm (after SRB) | < 0.4 µm | < 0.8 µm |
| Cylindricity | < 0.02 mm per 1,000 mm | < 0.03 mm per 1,000 mm | < 0.05 mm per 1,000 mm |
| Straightness | < 0.03 mm per 1,000 mm | < 0.05 mm per 1,000 mm | < 0.08 mm per 1,000 mm |
| Burr condition | None permitted | None permitted | None permitted |
| Hydrostatic test | 1.5× working pressure | 1.5× working pressure | 1.5× working pressure |
Inspection methods:
- Air gauging: Bore diameter at multiple depths and orientations
- Surface profilometer: Ra measurement at three positions along the bore
- Hardness test: Verification of barrel material hardness
- Borescope: Visual inspection of bore surface for defects
- Hydrostatic test: Pressure hold at 1.5× working pressure for 30 seconds minimum
- Liquid penetrant inspection: Weld area examination for base cap welds
Safety certifications: Elevator cylinders must comply with applicable elevator safety directives such as EN 81 (European), ASME A17.1 (USA), or equivalent national standards. These may require witnessed hydrostatic testing, material certification traceability, and third-party inspection of critical components.
Warning: Elevator cylinders are safety-critical components. A cylinder failure in service can result in the elevator car dropping — a potentially fatal event. All BTA-drilled and SRB-finished elevator cylinder bores must be 100 % hydrostatically tested at 1.5× working pressure with a hold time of at least 60 seconds. The test pressure must be recorded on a calibrated chart recorder, not a dial gauge. Any pressure drop during the hold period requires the cylinder to be rejected or subjected to additional NDT before re-testing.
Machine Configurations for Lift Cylinder Production
Elevator cylinder production requires machines capable of handling long, slender workpieces with large diameter bores.
Horizontal BTA machines for long cylinders:
The HTT-Global KT series and similar machines are well-suited for elevator cylinder barrels:
| Model | Drilling capacity | Boring capacity | Max depth | Spindle power | Coolant flow |
|---|---|---|---|---|---|
| KT100 | 20–80 mm | 25–100 mm | 10,000 mm | 11 kW | 250 L/min |
| KT200 | 40–80 mm | 40–200 mm | 10,000 mm | 30 kW | 400 L/min |
| KT350 | 40–80 mm | 40–350 mm | 10,000 mm | 30 kW | 600 L/min |
| KT500 | 40–120 mm | 40–500 mm | 10,000 mm | 37 kW | 1,000 L/min |
Combined BTA and SRB machines:
Integrated machines that perform both BTA drilling and skiving/burnishing in a single setup are common in high-volume elevator cylinder production. The workpiece remains in the same fixture for both operations, ensuring concentricity between the drilled and finished bore axes.
Vertical BTA machines for heavy cylinders:
For large freight elevator cylinders (250+ mm bore, 10+ metres length), vertical BTA drilling machines are used. The vertical orientation aids chip evacuation and allows gravity to assist with workpiece handling. The cylinder barrel is suspended from a lifting mechanism while the BTA head advances upward from below.
Automated handling for long-stroke cylinders:
Elevator cylinder production benefits from automated workpiece handling:
- Motorised roller conveyors for workpiece transfer between stations
- Automatic chip conveyor systems integrated with coolant filtration
- Programmable steady rests for long, slender barrel support during drilling
- Robotic deburring cells for port intersection deburring
Troubleshooting Lift Cylinder Drilling
| Symptom | Likely cause | Correction |
|---|---|---|
| Surface finish Ra > 0.4 µm after SRB | Burnishing overspeed — feed too high | Reduce SRB feed by 15 %; check burnishing roller condition |
| Seal leakage at < 50,000 cycles | Bore finish too rough in seal contact zone | Improve SRB finish to Ra < 0.2 µm; verify chrome plating of plunger |
| Ovality in long elevator cylinder bore | Coolant temperature drift during BTA cycle | Stabilise coolant at 25–30 °C; increase chiller capacity |
| Chrome plating peeling on plunger | Bore surface contamination before plating | Increase pre-plate cleaning; verify water break test |
| Plunger binding in cylinder assembly | Bore cylindricity out of tolerance | Verify SRB tool alignment; check steady rest positioning |
| Scored bore surface | Hard particle trapped between SRB tool and bore | Improve pre-SRB cleaning; increase coolant filtration to 25 µm |
| Excessive bore taper along length | Uneven coolant temperature from top to bottom | Add coolant circulation pump; improve thermal insulation of machine |
| Weld distortion at base cap | Weld shrinkage after final boring | Weld base cap before final SRB pass; use low-heat welding process |
| Short seal life in scissor lift cylinder | Side loading causing uneven seal wear | Increase bore straightness requirement; add wear ring groove |
| Coolant leakage past BTA pressure head | Ovality of barrel OD at entry | Verify OD roundness before drilling; use adjustable BOZA seal |
Frequently Asked Questions
What is the most common deep hole drilling method for elevator hydraulic cylinders? BTA drilling is the standard method for elevator cylinders with bore diameters of 50–290 mm. The large diameter and long stroke (up to 30,000 mm) make BTA drilling the most efficient method for material removal and bore straightness.
What materials are used for elevator cylinder barrels? C45 (AISI 1045) is the most common material for standard passenger elevator cylinders. 25CrMo4 (AISI 4130) and 42CrMo4 (AISI 4140) are used for heavy-duty freight elevators requiring higher strength.
How is the bore finished after BTA drilling for elevator cylinders? Skiving and roller burnishing (SRB) in a single pass is the standard finishing method. SRB achieves Ra 0.1–0.4 µm surface finish and H8–H9 diameter tolerance while work-hardening the surface by 10–20 %.
What surface finish is required for elevator cylinder bores? The standard requirement is Ra < 0.4 µm after SRB finishing. Premium elevators specify Ra < 0.2 µm. This surface finish is required for seal compatibility and long seal life over millions of operating cycles.
What is the typical cycle time for BTA drilling an elevator cylinder? A 120 mm × 4,500 mm passenger elevator cylinder requires approximately 3.6 hours for BTA drilling on a single-spindle machine. Adding SRB finishing adds 8–12 minutes. A 2-spindle machine halves the effective cycle time.
What coolant pressure is required for BTA drilling elevator cylinders? 7–40 bar depending on bore diameter. Larger bores (160–200 mm) require 7–15 bar while smaller bores (40–60 mm) require 25–40 bar. Coolant flow ranges from 150–1,200 L/min.
How does elevator cylinder drilling differ from agricultural or forklift cylinder drilling? Elevator cylinders are significantly longer (up to 30,000 mm vs 1,000–3,000 mm for forklift cylinders) with lower working pressure (30–100 bar vs 150–250 bar). The emphasis is on bore straightness and surface finish for seal compatibility rather than pressure containment. Chrome plating of the plunger (not the bore) is standard.
Is hard chrome plating of the bore necessary for elevator cylinders? No — for elevator cylinders, the plunger outer diameter is chrome plated, not the bore. The bore is finished by SRB. Chrome plating of the bore would be impractical for cylinders of this length and adds no benefit since the seal rides on the plunger OD.
What quality inspections are mandatory for elevator hydraulic cylinders? 100 % hydrostatic testing at 1.5× working pressure is mandatory. Bore air gauging, surface finish measurement, and straightness verification are standard. Safety certification compliance (EN 81, ASME A17.1) may require witnessed testing and material traceability.
What is the production volume for elevator cylinders? Major European elevator manufacturers produce 5,000–15,000 cylinder units per year for passenger elevators. Freight elevator and scissor lift volumes are lower at 500–3,000 units per year. Multi-spindle BTA machines are justified for passenger elevator volumes but not for specialised lift types.
Summary
| Aspect | Passenger elevator | Freight elevator | Scissor lift / platform |
|---|---|---|---|
| Typical bore diameter | 50–200 mm | 150–290 mm | 25–100 mm |
| Typical stroke length | 2,000–6,000 mm | 6,000–30,000 mm | 500–3,000 mm |
| Drilling method | BTA | BTA (vertical for very long) | BTA or gun drill |
| Typical material | C45, 25CrMo4 | 42CrMo4, 25CrMo4 | C45, ST52 |
| Cutting speed | 25–90 m/min | 25–65 m/min | 40–100 m/min |
| Feed | 0.10–0.30 mm/rev | 0.10–0.24 mm/rev | 0.04–0.12 mm/rev |
| Coolant pressure | 7–40 bar | 10–28 bar | 25–70 bar |
| Surface finish (SRB) | Ra < 0.4 µm | Ra < 0.4 µm | Ra < 0.8 µm |
| Bore tolerance | H9 | H8–H9 | H9–H10 |
| Hydrostatic test pressure | 1.5× working pressure | 1.5× working pressure | 1.5× working pressure |
Deep hole drilling for elevator and lifting equipment is a specialised segment of hydraulic cylinder manufacturing distinguished by extremely long stroke lengths, moderate bore diameters, and high surface finish requirements. BTA drilling followed by skiving and roller burnishing produces the precision bores required for reliable elevator operation over millions of cycles. The combination of long-stroke BTA machines with integrated SRB finishing capability and automated workpiece handling enables cost-effective production of elevator cylinders that meet stringent safety standards. As urbanisation continues to drive demand for elevators in low-to-medium rise buildings, particularly in Asia-Pacific and the Middle East, efficient deep hole drilling processes for elevator hydraulic components will remain essential to the lifting equipment supply chain.