Structure and Features
The NBB linear bushing consists of an outer cylinder,ball retainer,balls and two and rings.The ball retainer which holds the balls in the recirculating trucks in held inside the outer cylinder by end rings.
Those parts are assembled to optimize their required functions.
The outer cylinder is maintained sufficient hardness by heat treatment,therefore if ensures the bushing projected travel life and satisfactory durability.
The ball retainer is made from steel or synthetics resin.The steel retainer has high rigidity,obtained by heat treat meant.The synthetics resin retainer can reduce running noise.The user can select the optimum type for meeting the user's service conditions.
1. High Precision and Rigidity
The NBB linear bushing is produced from a solid steel outer cylinder and incorporates an industrial strength resin retainer.
2. Ease of Assembly
The standard type of NBB linear bushing can be loaded from any direction.Precision control is possible using only the shaft supporter,and the mounting surface can be machined easily.
3. Ease of Replacement
NBB linear bushings of each type are completely interchangeable because of their standardized dimensions and strict precision control.Replacement because of wear or damage is therefore easy and accurate.
4. Variety of Types
NBB offers a full line of linear bushing:the standard,integral single-retainer closed type,the clearance adjustable type and the open types.The user can choose from among these according to the application requirements to be met.
Types adn Linder Bushing Number
Note that precision of inscribed circle diameters and outside diameters for the clearance adjustable type (-AJ) and the open type (-OP) indicates the value obtained before the corresponding type is subjected to cutting process.
Load Rating and Life Expectancy
The list (L) of a linear bushing can be obtained from the following equation with the basic dynamic load rating and the load applied to the bush:
L:Rated life(km) fH:Hardness factor
C:Basic dynamic load rating(N) fT:Temperature coeffcient
P:Working load(N) fC:Contact coefficient
The lifespan(Ln) of a linear bushing in hours can be obtained by calculating the travelling distance per unit time.
The lifespan can be obtained from the following equation if the stroke length and the number of strokes are constant:
Lh:Lifespan(hr) s:Stroke length(m)
L:Rated life(km) n:Number of strokes per minute(cpm)
Relation between ball circuits and load rating
The NBB linear bushing includes ball circuits that are spaced equally and circumferentially.The load rating varies according to the loaded position on the circumference.
The value in the dimension table indicates the load rating when the load is placed on top of one ball circuit.If the NBB linear bushing is used with two ball circuits loaded uniformly,the load rating will be greater.The following table shows the values by the number of ball circuits in such cases:
1. Obtaining the rated life L and lifespan Lh of the NBB linear bushing used in the following conditions:
Linear bushing: LM20UU
Stroke length: 50mm
Number of strokes per minute: 50cpm
Load per bush: 490N
The basic dynamic load rating of the linear bushing is 882N from the dimension table.From equation(1),therefore,the rated life L is obtained as follows:
From equation(2),the lifespan Lh is obtained as follows:
2. Selecting the linear bushing type satisfying the following conditions:
Number of linear bushing used: 4
Stroke length: 1m
Traveling speed: 10m/min
Number of strokes per minute: 5cpm
Total load: 980N
From equation(2),the traveling distance within the lifespan is obtained as follows:
From equation(1),the basic dynamic load rating is obtained as follows:
Assume the following with a pair of shafts each with two linear bushings:
As a result,Lm30 is selected from the dimension table as the NBB linear bushing type satisfying the value of C
Clearance and Fit
When a standard-type NBB linear bushing is used with a shaft,inadequate clearance,adjustment may cause early bush failure and/or poor,rough travelling.The clearance adjustable linear bush and open linear bush can be clearance adjusted when assembled in the housing which can control the outside cylinder diameter.However,too much clearance adjustment increases the deformation of the outside cylinder,to affect its precision and life.Therefore,the appropriate clearance between the bush and shaft,and clearance between the bush and housing are required according to the application.Table 2 shows recommended fit of the bush:
Shaft and Housing
To optimize performance of the NBB linear bushing high precision of the shaft and housing is required.
The rolling balls in the NBB linear bushing are in point contact with the shaft surface.Therefore,the shaft dimensions,tolerance,surface finish,and hardness greatly affect the traveling performance of the bush.The shaft should be manufactured with due attention to the following points:
1) Since the surface finish critically affects smooth rolling of balls,grind the shaft at 1.5 S or better
2) The best hardness of the shaft is HRC 60 to 64.Hardness less than HRC 60 decreases the life considerably,and hence reduces the permissible load.On the other hand,hardness over HRC 64 accelerates ball wear.
3) The shaft diameter for the clearance adjustable linear bush and open linear bush should as much as possible be of the lower value of the inscribed circle diameter in the specification table.Do not set the shaft diameter to the upper value.
4) Zero clearance or negative clearance increases the frictional resisiance slightly.If the negative clearance is too tight,the deformation of the outside cylinder will become larger,to shorten the bush life.
There is a wide range of housing differing in design,machining ,and mounting.For the fitness and shapes of housings,see Table 2 and the following section on mounting.
When inserting the linear bush into the housing.do not hit the linear bush on the side ring holding the retainer but apply the linear bush into the housing by hand or lightly knock it in. In inserting the shaft after mounting the bush,be careful not to shock the balls.Note that if two shafts are used in parallel,the parallelism is the most important factor to assure the smooth linear movement.Take care in setting the shafts.
Examples of Mounting
The popular way to mount a linear bush is to operate it with an appropreiate interference.It is recommended,however,to make a loose fit in principle because otherwise precision is apt to be minimized.The following examples show assembling of the inserted bush in terms of designing and mounting,for reference.