For a tie rod, ground bar is usually the better starting point: the rod is a tension member, and its ends still have to be threaded or machined. For a guide rod that slides in a bushing, a bearing or a seal, a chrome-plated bar is normally the right call, because the surface itself is the wear part. The two products start from the same ground bar; the difference is the hard chrome layer and what it is asked to do.
What does chrome plating actually change on a ground bar?
A chrome-plated bar is a ground bar with a hard chrome layer on top. The chrome changes the surface; it does not change the strength of the section.
The bar is cold drawn or peeled by diameter, roll-straightened, centerless ground and polished, then hard chrome is applied by a long-term partner plating plant to Yixin's specification, and the bar is polished again. On Yixin bars the hard chrome is 10–50 μm per side, chosen per order, and chrome hardness can reach HV800+. Surface finish is Ra ≤ 0.4 μm on chrome, against Ra ≤ 0.8 μm on ground bar.
What that buys is a hard, smooth running surface. What it does not buy is tensile strength, bending strength or fatigue performance: those come from the grade and the heat treatment of the bar underneath. A tie rod that only pulls, sits indoors and is never touched along its length gets little from a chrome layer except a different appearance. A rod that rubs, sees condensation, or is exposed on a machine gets a great deal.
When is ground bar the better choice for a tie rod?
Ground bar suits a tie rod whose ends will be threaded, turned, drilled or milled after delivery, because there is no hard layer to interrupt at the machined zone.
The usual case is a tension member cut to length, threaded at both ends and fitted with a nut, clevis or turnbuckle. Cutting a thread through a hard chrome layer means the plated surface at the thread root is removed anyway, so the layer is paid for and then machined away. On a ground bar the section at the thread is the same material as the rest of the rod, and the diameter you ordered is the diameter you machine from.
Yixin's 45 Steel Ground Round Bar covers a standard range of Ø4–140 mm, with up to Ø200 mm made to order. Where the tie rod carries more than a light load and the drawing calls for a stronger base, the 40Cr Q&T Ground Round Bar covers Ø5–140 mm with a Q&T hardness of HRC28–35, and its ends can still be machined in your shop.
Ground bar is also the practical answer when the fit at the ends is tight and you would rather cut the final tolerance yourself than work around a plated surface.
When does a guide rod need a tie rod chrome bar?
A guide rod that slides — in a bronze bushing, a linear bearing or through a rod seal — should carry a hard chrome surface, because the surface is the part that wears.
In that duty the rod is not only pulling; it is being rubbed, side-loaded and exposed. A hard chrome layer of 10–50 μm per side with a chrome hardness that can reach HV800+ gives the running surface a wear resistance that the base steel does not have, and the polished chrome finish of Ra ≤ 0.4 μm presents a smoother face to a seal or a bearing than a ground surface does.
The product choice then depends on how the rod is guided:
- For a rod running in a linear bearing, the 45 Steel Chrome-Plated Linear Shaft is made to the fits those bearings expect: Ø4–11 mm at −0.02/−0.05 mm, and from Ø12 mm at 0/−0.02 mm typical for linear bearings, within a Ø4–130 mm range.
- Where the surface must resist indentation as well as wear, the 45 Steel Induction-Hardened Chrome Shaft adds a case of at least HRC55, about 2 mm deep, under the chrome.
- Where the whole section carries load, a Q&T chrome-plated bar is the better base, and the 42CrMo Q&T Chrome-Plated Bar is intended for heavy or shock loads.
Which base grade and condition should the bar be in?
Pick the base from the load and the heat treatment from the section size, then decide whether the surface needs chrome.
Grade 45 is the default for light duty. In the 45 Steel Chrome-Plated Soft Shaft it is not induction hardened, at about HRC17. The 45 Steel Q&T Ground Round Bar and 45 Steel Q&T Chrome-Plated Bar are quenched and tempered to a surface hardness of HRC25–28 or HRC28–32, chosen per order; because grade 45 has limited hardenability, the core stays softer, so that figure should not be read as through hardness.
The 40Cr Q&T grades reach a Q&T hardness of HRC28–35 and are the step up for loaded guide components and transmission parts. The 42CrMo Q&T Chrome-Plated Bar, at a Q&T hardness of HRC30–38 and Ø8–100 mm made to order, is for chrome shafts and piston rods under heavy or shock loads.
Grade names follow Chinese standards: Grade 45 to GB/T 699-2015, and 40Cr and 42CrMo to GB/T 3077-2015. Comparable grades such as C45, S45C or SAE 1045 for 45 steel, 41Cr4 or SCr440 for 40Cr, and 42CrMo4 or SCM440 for 42CrMo are for reference only; chemistry limits differ between standards.
| Bar | Base and condition | Surface | Fits |
|---|---|---|---|
| 45 Steel Ground Round Bar | 45 steel, not Q&T, about HRC17 | Ground, Ra ≤ 0.8 μm | Tie rods and machined shaft parts with light surface duty |
| 40Cr Q&T Ground Round Bar | 40Cr, Q&T hardness HRC28–35 | Ground | Loaded tie rods and guide parts whose ends are machined after delivery |
| 45 Steel Chrome-Plated Linear Shaft | 45 steel, induction hardened, case at least HRC55 | Hard chrome | Guide rods running in linear bearings |
| 45 Steel Q&T Chrome-Plated Bar | Q&T surface HRC25–28 or HRC28–32 | Hard chrome | Chrome rods where the section carries moderate load |
| 42CrMo Q&T Chrome-Plated Bar | 42CrMo, Q&T hardness HRC30–38 | Hard chrome | Chrome rods under heavy or shock loads |
How thick should the chrome be, and how do I control it?
Yixin applies hard chrome at 10–50 μm per side, chosen per order, so the figure belongs on the drawing together with the finished diameter and its tolerance.
The two are stated separately and confirmed together on the drawing and order: the finished diameter you need, and the chrome thickness you want on it. Where a heavier layer is called for, the lead time is quoted per order. For tight fits, say clearly what the tolerance applies to, and give the tolerance band you want from the options that fit your diameter — for example 0/−0.02 mm, −0.01/−0.03 mm or −0.02/−0.05 mm from Ø12 mm.
On corrosion, be careful what you ask for. Salt-spray performance is not part of the standard specification. Where a rating is required, we specify a thicker chrome layer and can have the order tested by a third-party laboratory.
What to put in your RFQ
- Say whether the part is a tie rod in tension or a guide rod with a sliding surface, and where it slides.
- State the grade and condition you want — 45 steel (soft, Q&T or induction hardened), 40Cr Q&T, or 42CrMo Q&T — with the GB standard on the drawing.
- Give the finished diameter and tolerance for each diameter on the rod, and the chrome thickness in μm per side if you have a figure in mind.
- Mark the unplated zones at threads, flats and clamped areas, or state that the ends will be machined after delivery.
- Give straightness, surface finish Ra and the cut length, and say whether ends are to be left as sawn.
- State the quantity, remembering that the minimum is 1 t per order and different sizes can be combined, and the full bar lengths you can handle (4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm).
- Name the destination port for FOB Shanghai or Ningbo, and any packing marking you need.
Summary
The decision follows the function, not the appearance. A tie rod that pulls and is machined at both ends is best served by ground bar, in 45 steel for light duty or 40Cr Q&T where the load is higher. A guide rod that slides needs a chrome-plated bar, with the base chosen for load and the chrome layer set on the drawing. When in doubt, state the duty, the diameter and tolerance, the chrome thickness in μm per side, and the unplated zones — and let those four lines drive the quotation.
References
- GB/T 699-2015 — the standard to which Yixin's Grade 45 is ordered.
- GB/T 3077-2015 — the standard to which Yixin's 40Cr and 42CrMo are ordered.