Sliding bearings

Machine type sliding bearings

How are fine (precise) design machine type sliding supperts are made?

a) moving pivot (stationary bushing),

b) stationary pivot (moving bushing).

Machine type slinding bearings features

\- circumferential speed of the pivot is very small, up to only 70 / 80 mm/s

\- specific pressure is insignificant

How are the simplest bearings are made?

are made in the form of a hole in the housing or frame .

Machine type sliding bearings – moving pivot

How shafts are typically made?

Shafts are often made from calibrated rods and the pivots are not machined (fig. a). Turned pins (fig. b) are made with roughness Ra = 1,251,6 mu m, grounded ones Ra = 0,8 mu m.

how are housings must be made?

Housings which are simultaneously bearing plates must be made of proper material to made with the steel a good pair ( with the smallest coefficient of friction). Therefore often are made of brass CW508L (old M63), the CW509L (old M60) or, in the case of bodies of cast brass CB612P (old MO59).

When is it necessay to use bearing blocks / bushes?

**If the body is made of:**

**• materials which do not form a good frictional pair with steel , for example. light alloy (silumin AK11 AB-44100), ceramics, plastics, etc. • thin sheet metal**

From what materials are the bushings made off?

Bushings are made of materials that ensure good cooperation with steel pin, i.e. bronze CW456K (old B443) or brass CW617N (old MO58)

Types of bushes

* pressed in
* screwed
* mould inserted
* riveted
* fastened

Pressed-in bushes

Screwed bushes

Mould-inserted bushes

1 – bushing (metal insert) , 2 – moulded plastic wall

Riveted bushings

Fastened bushes

Bearing bushes fastened by a screws to : a) a metal plate , b) moulded plastic wall c) metal plate / plastic wall 1- bush, 2 – housing wall, 3 – screw, 4 – metal plate, 5 – locking nut

How are Bearing bushes fastened by a screws to :

a) a metal plate , b) moulded plastic wall c) metal plate / plastic wall

Machine type sliding bearings – stationary pivot

Pins are secured by : a, b, c, d, e, f, i, j) – fasteners , g) – point welding, h) – kneading

how are pins secured in stationary pivot

* fasteners
* point welding
* kneading

Types of sliding bearings in stationary pivot

Clock type sliding bearings

Direct bearings in mechanism plates

clock type bearing with conical recess to retain lubricating oil

Clock type bearing, friction torque

Clock type bearing, chamfer functions bita = 45 degrees

* reduction of friction torque caused by axial load Q,
* prevents the grease from outflowing (capillarity/wicking),
* serves as a basis for the manufacturing of the pinion teeth

reduction of friction torque caused by axial load Q

Diameters D, d i D1 have to be made allowing existence of the face.

Clock type bearing, Materials

Pin: steel 10S20 (old A11) plate: brass CW508L (old M63) brass CW509L (old M60) Pin diameters : 0,1 up to 3 mm, Pin bushing fits: H9/d9, H9/cd9

How are the bushings made when the plates are thin?

bushings are made in a form of inserts.

MINERAL BUSH (JEWEL) BEARINGS FITTED INTO MECHANISM PLATES

Clock type bearing, mineral bushes - materials

Corundum: dyed red or pink, grind-able.

Chalcedony: porous and dyed (red, brown).

Clock type bearing, mineral bushes - design types

(a) with rounded (olive shaped) hole,

(b) flat, pressed-in,

(c) rounded, with German (Glashütte) type mounting

(d) rounded, with Swiss type mounting,

(e) flat, with Swiss type mounting

with rounded (olive shaped) hole,

flat, pressed-in

rounded, with German (Glashütte) type mounting

rounded, with Swiss type mounting

flat, with Swiss type mounting

Clock type bearing, mineral bushes - features

* Pin nominal diameter from 0,1 up to 4 mm,
* pin material: steel 10S20 (A11), Ra = 0,08 micro m (burnished, roll-polished),
* bush ID tolerance H4 / H8,
* bush OD tolerance : h7,
* extremely small drag, small wear, long endurance, lubricated during assembly,
* usage: precise bearings(supports) in measuring equipment

Porous – metal bushes slide bearings;

sintered bushes

sintered bushes

Most porous-metal bearings consist of either bronze or iron which has interconnecting pores. These voids take up to 10 to 35% of the total volume

what is the operational purpose of the porous metal bearings

In operation, lubricating oil is stored in these voids and feeds through the interconnected pores to the bearing surface. Any oil which is forced from the loaded zone of the bearing is reabsorbed by capillary action. Because these bearings can operate for long periods without additional lubricant, they can be used in inaccessible or inconvenient places where re-lubrication would be difficult.

Porous – metal bearings

Pressed-in bush with additional lubrication

Self-aligning bushes with additional lubrication

Types of porous metal bearings

* Pressed-in bush with additional lubrication
* Self-aligning bushes with additional lubrication

Porous – metal bearings; recommended fits

Pressed-in bushes are located by fit H7/r7 or H8/r8 Hole basis fit (H) is beneficial due to the easy selection of reamers designed to comply with holes.

Porous – metal bearings; features

Pin features recommendations:

\- higher grade carbon steel (C35, C55)

\- Surface roughness Ra = (0,04 / 0,08) microm !

\- hardness at less 50 HRC, when sintered iron powder bushes are used

Oil features recommendations :

\- Working temperature range from –12 up to +900 C,

\- viscosity at 500 C:

3 / 5 E (Engler degree) or 20  35 cSt ( centi-Stokes)

Working limits of porous metal bearings

• maximum slide velocity vmax = (610) m/s,

• maximum specific pressure pspc = (1020) MPa,

• maximum static load, that does not cause the deformation of a diameter of more than 0.1%, pmax = (50 100) MPa,

• the limit value of (p·v)max (1,81,6) MPa·m/s

Porous – metal bearings; usage

• low power electric motors,

• mechanized household appliances,

• power tools,

• machines for textile, food industry

• endoprosthesis,

• high speeds at low and medium loads,

Sintered silicon carbide bushes

Silicon carbide is one of the hardest known materials. This material apply due to its characteristic properties.

Properties of sintered silicon carbide:

• very high hardness (2500 HV)

• excellent abrasion resistance,

• very good chemical resistance and mechanical strength. up to 2000° C,

• high thermal conductivity, resistance to thermal shocks (Δ \~ 250° C)

• low thermal expansion (4.3 / 5.8 x 10-6 K),

• low density (approximately 3.1 g/cm3 )

Polymer plain bearings

Polymer bearing bushes may be made of fabric reinforced laminates or polyamides. Bushes made of some of these materials may be impregnated with lubricant before assembly. Consequently, such supports are lubricated for life. Polymer bushes have a low coefficient of friction and a high resistance to wear (higher than that of bronze bushes).

Polymer plain bearings

As plastics have a low thermal conductivity and heat resistance, they may not be used for heavily loaded supports working at high rotational speeds or the large amount of heat generated by friction would cause them to be damaged. Specific loads for these bearings should not exceed 0,1 MPa

General features of polymer plain bearings

• Lubricant-free

• Corrosion resistance

• Maintenance-free

• Media resistant

• High compressive strengths

• Low coefficients of friction

• High mechanical dampening

• maintenance-free

• High dirt resistance

• Lightweight

• Best wear-resistance

• Very good price/performance ratio

Disadvantages of polymer plain bearings:

Absorb moisture from the environment (hygroscopy). I.e water absorption: polyamide 6 ÷10% polymethanals (PFA) 1.5%, PTFE-0%

\- are dimensionally unstable over time:

• different casting contraction,

• structural changes in the material,

• ability of hygroscopy

Disadvantages of polymer plain bearings:

\- low thermal conductivity restricts the carrying capacity of the bearings:

• lower values of unit pressure plimit,

• unfit to work at high speeds to slip v

• Permitted p∙v value is crucial

\- mechanical properties changes over time (the aging of the material)

\- unsuitable for work at higher temperatures (e.g. lowered hardness)

\- harder to maintain lubricant, which means they are more likely to work on dry - the unacceptable pair: plastic-brass

What is p∙v product ?

The p∙v value can be seen as a measurement of frictional heat and can therefore be used as an analytical medium to justify the applicability of a bearing. For this purpose the actual p∙v value is compared with a permitted p∙v value calculable for the height. The permitted p∙v value is dependent on the material of the glide partner, ambient temperature and the on-time. „

Permitted p∙v value

K1 , K2 Constant for heat conductivity (K1 = 0,5; K2 = 0,042)

s Bearing wall thickness in mm

b1 Bearing length in mm

μ Coefficient of friction

λs Heat conductivity of the shaft

λk Heat conductivity of the bearing

ΔT (Ta - Tu ) where

Tu - ambient temperature;

Ta - max. ambient temperature

Permitted p∙v; correction factors

The permitted p∙v value can be raised in the intermittent service, if the bearing temperature has not at all reached the maximum due to the short turn-on times. Tests have shown that this is the case in turn-on times under 10 minutes. The shorter the turn-on time, naturally lower is the highest bearing temperature attained

Polymer plain bearings Limits of working conditions:

\- permitted value p∙v:

* from 0,3 N/mm2 \* m/s PA6.6


* up to 3,5 N/mm2 \* m/s iglidur X,

\- maximum working temperature 100  2500 C

\- maximum slide velocity v = 3 / 5 m/s,

Polymer plain bearings Pin quality recommendations:

\- High grade carbon steel,

\- Surface roughnes Ra = (0,4 / 0,63) micro m,

\- Hardness c.a 50 HRC

Both worse roughness and greater smoothness cause wear increase !

Prismatic (V-block) sliding bearings